Well you might as well pay the extra tenner and have the full 30 minutes with them.
Personally I don't pay for any longer than I need to ejaculate. I've learnt over time that this is pointless
I can cum quite easily in 15 mins so I'd rather keep the extra £10 in my pocket than pay £10 to be in a prossies company for 15 extra mins
I'll only ever do a 30 min punt nowadays if the girl does not have a 15 min rate AND I fancy her