I've done mine ranging anywhere from a few days to four months and more after the original review. I feel like, if I have something new to add, I should add it as a follow-up review so that people will be more likely to notice it than if I just slip it into the original review somewhere. I also tend to edit them periodically if there's something else I want to say as a second follow up or an update to the original. I've followed up to say that I've broken the toy, that I found some new information on it or the materials...
Actually, on one review I did on edible underwear, I followed up with just some humorous stuff about different types of edible panties I'd found online...Homemade ones! Bacon, fruit roll-ups, etc. I figured, in that case, there wasn't much I could really say in the way of a "real" follow-up, so I just wanted to add something entertaining
Also, there've been a few times where I reviewed a toy that I had already had for a fairly long time, so I tried to stick mostly to my original impressions of the toy for the main review. Then in the follow-up I would add in my thoughts on it as they are now and what I learned as time went by and I got to know the toy better.
It all depends, I suppose...I think the two month reminder is great, but I think there are some cases where it's acceptable to update with a follow-up shortly after the original review.