Ivan,
That's a difficult question because it depends on how "difficult" your customer is. I used to work as a PM for a software development company. The minimum that we used to add for PM work was 30%. But there was this one customer that I had where I regularly added 40% of PM work because they really, really needed more help.
Of course, then there are other areas when you can go much lower. For instance: during the time when actual coding got done, our lead developer would manage the team and my involvement would go down to 10%. But during testing, the PM percentage would go back up to 30% because I needed to be involved and communicate with the customer.
From that I learned that it's best not to talk about a "fixed" percentage, but to look at each "segment" of the project itself, determine how much support the customer really needs (lessons learned are important here) and calculate the PM time accordingly.