Every time they raise investment, founders are trading off between dilution and the amount and value of the funding round. The oft used adage is that it's better to own a smaller slice of a bigger pie, than lots of a small pie - but whenever you give away shares to new investors, you don't know if the growth in value will make up for the dilution. 

Blossom Street Venture have published some great data on how much equity successful founders hold at exit. The median final ownership stake for a founder is 11%, and there's a big range behind that. Some founders, like Mark Zuckerburg or Jonathan Oringer end up over 50%, while there are plenty down around 5%.

Clearly, there's no right answer to how much dilution is too much. A lot comes down to individual circumstance. Dilution also isn't the only factor to worry about, the terms of investments can have just as much impact on a founder's return as the absolute level of dilution.