Okay, finally figure out what to grep for:

2018-05-17 08:44:52 INFO juju.txn prune.go:128 txns after last prune: 251800, txns now: 357278, pruning: too many new transactions
2018-05-17 11:01:17 INFO juju.txn prune.go:128 txns after last prune: 251800, txns now: 778357, pruning: too many new transactions
2018-05-17 11:03:09 INFO juju.txn prune.go:153 txn pruning complete after 1m52.040008885s. txns now: 274117, inspected 63 collections, 27332 docs (2077 cleaned)
2018-05-17 12:03:09 INFO juju.txn prune.go:128 txns after last prune: 274117, txns now: 531412, pruning: too many new transactions
2018-05-17 12:04:30 INFO juju.txn prune.go:153 txn pruning complete after 1m20.513413195s. txns now: 263121, inspected 63 collections, 20481 docs (6564 cleaned)
2018-05-17 13:04:30 INFO juju.txn prune.go:128 txns after last prune: 263121, txns now: 520497, pruning: too many new transactions
2018-05-17 13:05:36 INFO juju.txn prune.go:153 txn pruning complete after 1m5.471639408s. txns now: 262156, inspected 63 collections, 14467 docs (434 cleaned)
2018-05-17 14:05:36 INFO juju.txn prune.go:128 txns after last prune: 262156, txns now: 519253, pruning: too many new transactions
2018-05-17 14:06:31 INFO juju.txn prune.go:153 txn pruning complete after 55.140586434s. txns now: 261032, inspected 63 collections, 14479 docs (446 cleaned)
2018-05-17 15:06:31 INFO juju.txn prune.go:128 txns after last prune: 261032, txns now: 519604, pruning: too many new transactions
2018-05-17 15:07:35 INFO juju.txn prune.go:153 txn pruning complete after 1m3.926678727s. txns now: 263312, inspected 63 collections, 19079 docs (451 cleaned)
2018-05-17 16:07:35 INFO juju.txn prune.go:128 txns after last prune: 263312, txns now: 520881, pruning: too many new transactions
2018-05-17 16:08:28 INFO juju.txn prune.go:153 txn pruning complete after 52.362132882s. txns now: 261125, inspected 63 collections, 16821 docs (1175 cleaned)
2018-05-17 17:08:28 INFO juju.txn prune.go:128 txns after last prune: 261125, txns now: 518994, pruning: too many new transactions
2018-05-17 17:09:23 INFO juju.txn prune.go:153 txn pruning complete after 55.783720751s. txns now: 261729, inspected 63 collections, 14771 docs (539 cleaned)
2018-05-17 18:09:24 INFO juju.txn prune.go:128 txns after last prune: 261729, txns now: 519350, pruning: too many new transactions
2018-05-17 18:10:26 INFO juju.txn prune.go:153 txn pruning complete after 1m2.719207323s. txns now: 262231, inspected 63 collections, 14660 docs (464 cleaned)
2018-05-17 19:10:28 INFO juju.txn prune.go:128 txns after last prune: 262231, txns now: 520676, pruning: too many new transactions
2018-05-17 19:11:26 INFO juju.txn prune.go:153 txn pruning complete after 58.66493666s. txns now: 262614, inspected 63 collections, 15923 docs (492 cleaned)
2018-05-17 20:11:27 INFO juju.txn prune.go:128 txns after last prune: 262614, txns now: 520286, pruning: too many new transactions
2018-05-17 20:12:25 INFO juju.txn prune.go:153 txn pruning complete after 58.163812471s. txns now: 261906, inspected 63 collections, 15726 docs (706 cleaned)
2018-05-17 21:12:25 INFO juju.txn prune.go:128 txns after last prune: 261906, txns now: 519117, pruning: too many new transactions
2018-05-17 21:13:26 INFO juju.txn prune.go:153 txn pruning complete after 1m0.888025344s. txns now: 261581, inspected 63 collections, 14761 docs (557 cleaned)
2018-05-17 22:13:26 INFO juju.txn prune.go:128 txns after last prune: 261581, txns now: 519084, pruning: too many new transactions
2018-05-17 22:14:25 INFO juju.txn prune.go:153 txn pruning complete after 59.219452113s. txns now: 261947, inspected 63 collections, 14576 docs (435 cleaned)
2018-05-17 23:14:25 INFO juju.txn prune.go:128 txns after last prune: 261947, txns now: 519450, pruning: too many new transactions
2018-05-17 23:15:24 INFO juju.txn prune.go:153 txn pruning complete after 59.303205079s. txns now: 261820, inspected 63 collections, 14511 docs (457 cleaned)
2018-05-18 00:15:24 INFO juju.txn prune.go:128 txns after last prune: 261820, txns now: 519367, pruning: too many new transactions
2018-05-18 00:16:18 INFO juju.txn prune.go:153 txn pruning complete after 53.51410596s. txns now: 261466, inspected 63 collections, 14509 docs (431 cleaned)
2018-05-18 01:16:18 INFO juju.txn prune.go:128 txns after last prune: 261466, txns now: 518683, pruning: too many new transactions
2018-05-18 01:17:16 INFO juju.txn prune.go:153 txn pruning complete after 57.198467444s. txns now: 261113, inspected 63 collections, 14540 docs (454 cleaned)
2018-05-18 02:17:16 INFO juju.txn prune.go:128 txns after last prune: 261113, txns now: 518789, pruning: too many new transactions
2018-05-18 02:18:21 INFO juju.txn prune.go:153 txn pruning complete after 1m4.740267702s. txns now: 262378, inspected 63 collections, 14475 docs (442 cleaned)
2018-05-18 03:18:21 INFO juju.txn prune.go:128 txns after last prune: 262378, txns now: 519477, pruning: too many new transactions
2018-05-18 03:19:29 INFO juju.txn prune.go:153 txn pruning complete after 1m7.800545051s. txns now: 262114, inspected 63 collections, 14498 docs (424 cleaned)
2018-05-18 04:19:29 INFO juju.txn prune.go:128 txns after last prune: 262114, txns now: 313918, pruning: transactions have grown significantly
2018-05-18 04:19:44 INFO juju.txn prune.go:153 txn pruning complete after 15.410154434s. txns now: 258670, inspected 63 collections, 13915 docs (124 cleaned)