Well, I'm also on the text heavier side of things (a weak spot I have to improve upon), so take what I say with a grain of salt.
I think it depends on the text, first of all. Is the text telling us something we're already seeing? Is it redundant? Is the panel better without that text?
Without context is hard to say. One thing about webcomics is that their layouts and the space you have is highly flexible, so there's always possibility to spread your text over a ''wider canvas''.
But as a rule of thumb, most of times I like to follow something I read about a Kelly Sue Deconnick workshop:
Max of 210 words per page (Moore)
Max of 3 lines per balloon (Brubaker)
Max of 3 balloons per panel (Ellis)
This is about printed comics, though, with a fixed page size. That said, I think it's a great rule fo thumb for ''western comics'' (manga tends to have a more relaxed and decompressed pacing, so if you're not One Piece or Death Note, chances are there's a lot less text in a page).
As with everything, it's a general guideline that can and must be broken, should necessity arise. I myself don't quite agree with the 3 lines one, so I tend to ignore it.