It would probably be 20-25% for me to quit right now and join in a month. However, a lot of this will depend on who is offering the money, and the quality of the job I'll be walking into.
Let's say Company X offer me a 20% increase on my salary, but developers at Company X are stressed out and work tons of extra hours, I'd flat out decline. If Company Y come along, offer me 5% more money than I am currently paid, but have a much more relaxed atmosphere then I would probably take it.
I've had a few offers from companies that would bump my salary up about 25%, but I know people that have worked for these companies, and they have awful reputations for treating their developers like shit.
But if you were lowballed at your current job, aim for the top of the range! Look up your position on glassdoor.com and see what you're really worth. Information is ammunition!