There is no set time on how long it should take, every window cleaner works at a different speed!! Without talking about hourly pay etc etc, if it takes you 30 mins per property for an excellent finish then thats 2 an hour at least so it can't be too bad!!! if they are well priced. Its better to take your time than rush and do a rubbish job, it took me well over a year to get my speed up both trad and wfp, been at it a while now and i am quick but i think about the finish more than the time it takes!!!
Just stick with it and you will speed up.