This is very important. Sometimes women put too much pressure on themselves and suffer their own version of performance anxiety. If they don't orgasm after 5 or 10 minutes, they become preoccupied with thoughts of whether they have a problem, or whether you perceive them as having a problem, etc.). If they are thinking about other things, orgasm is going to be difficult to achieve.
Start off by telling her how much you love touching her (message - I get pleasure out of this too). Then tell her you'd like to spend a little more on this kind of foreplay (message - I'm going to be there a while). Then ask her if this is ok, even though she may not finish, because you like it (message - this isn't about whether you finish or not). This may take the pressure off.