I think it should be when the user has actually used the system for an amount of time, with other heuristics such as how often they use it, what time you ask for a donation etc. Then you could apply statistics and A/B testing to see what combination of factors maximises donations.
I think it should be when the user has actually used the system for an amount of time, with other heuristics such as how often they use it, what time you ask for a donation etc. Then you could apply statistics and A/B testing to see what combination of factors maximises donations.