Dunno about Keeley and nobody has posted a link but if she's a good girl as a couple of peeps have said then that will make it easier.
Generally pay first
I would say the time is when you walk in BUT if she asks you to have a shower (which you should do immediately before going if possible) I would suggest you accept and assume the time starts from when that finishes - just don't hang around in there. Most decent girls seem to be able to pace a session - or more to the point if time is running out you'll get asked how you want to finish. That's your cue there's 5-10 minutes left.
At the start of the session you might face "the test". She might ask if you want a massage - for escorts a massage is code for squat over your face and have a dump in your mouth. Always decline or possibly "maybe later".
If she offers services you might like bear in mind she isn't the sweet girl who works in the florist so if you were to for example ask "can I spunk on your tits and watch you lick it off" she will likely not be offended or embarrassed so neither should you be. You probably won't be the first or last person to ask - that day even. Of course don't push on anything she's uncomfortable with.
Bear in mind who is paying you so - it's nice for the girl to enjoy herself but it's your birthday party. If she offers to go on top but you want to go on top say so. if you then decide you want her to go on top - say so. If you then decide.... your party capiche.
Did I mention not being embarrassed about asking for what you want - go read that again - about 100 times.
If you get nervous and things go a bit - soft - well you are in bed with a gorgeous babe (haven't checked she might be size 20 and 67 years old) you aren't there to impress her, she's seen it all before and I'm sure there's something more fun to do than getting stressed. you could ask for a massage even - it's ok if you ask, no squatting.