My method is simple. The first time I ask myself "I wonder if I should change these strings?..." I just change 'em.
The exception is just before recording or performance. I tend to do those swaps at least a day or two in advance to aviod string stretching issues making me crazy.
Nylon strings can go a surprisingly long time without a change, but that doesn't mean they should. A large percentage of some of the most basic uke issues (intonation, buzz, volume, etc...) can be fixed with a simple string swap.