After several years of experience? I know you won't start off too high (I hear like $35-$45K) but what about as you advance?
It depends on the nonprofit you're working for. There's a good chance that your salary won't increase at all.
This is really not true in most places. Most non-profits have step increases of about 7-10% per year, usually capped somewhere in the high $80s. Some major national non-profits are keyed to the DOJ pay scale, which is considerably higher.
The majority of nonprofit law positions in this country do not give a 7-10% increase per year. That's insane. While high profile nonprofit law positions might, the overwhelming majority of nonprofit law positions in this country (to take just one example, working for a domestic violence shelter) do not. Most nonprofits scrape by on tiny budgets provided by grants that need to be renewed yearly. Within these nonprofits there is almost zero room for advancement, as there may only be one legal position.
Oh, I'm sorry. I thought that we were discussing legal non-profits (like legal services organizations), which do, indeed, have typical increases in this range. Of course, this is not true at all organizations, but, in general, staff attorneys protect their salary increases in times of cuts. For perspective, this is starting at like $45,000, moving up to $48,500, then up to $51,750, etc.