I stopped watching at around mid Johto, then came back during DP, stopped at BW, and came back for a bit of XY but then stopped again.
Like Meloetta, I pretty much would watch depending on my level of overall interest in Pokemon. These were my major spikes of interest and most frequent periods of playing the games. But honestly, by now, I've pretty much realized the anime isn't worth watching. Everytime I jump back in, after watching a few episodes in, I get bored by how repetitive and boring the series is. There's barely any character development, the voice actors (of the English dub, and no, I don't want to watch it subbed) are awful, and the plot plays it so safe that there really isn't an ongoing conflict for me to care about. It's just a vague continuous goal that never ends for Ash with no hope of closure in sight. Something that, for how lengthy it is, is not worth getting invested in in my opinion.
I'll pretty much only pop in to watch an episode if it's particularly interesting or has my favorite Pokemon in it. (For instance, I watched the episode of XY where Braixen loses her stick because I love Braixens and found it to be adorable.)