Earlier on was only a couple of years if I remember correctly (obviously my time messing with Neopets is a little fuzzy hardly a core memory!)especially once it was acquired by Viacom.

Did a cursory search so take all this with a grain of salt, but looking at the timeline of when ads are introduced, then the acquisition, peak users, etc. I’d say most people were playing in a pretty serious corporate sandbox for most of its most relevant years.