It's up to you. On the one hand, it's been running for how long with only random voltage fluctuations. On the other hand, any part can fail at any time; the difference in this case is that the PSU can take other parts with it, as opposed to say RAM or a HDD, which would just cause performance issues. As far as sooner vs later, it all depends on how much of a chance you want to take. Personally, if it were me and I had the means to replace it, I'd opt for sooner.
The importance of a name-brand, quality PSU cannot be understated.