I like that ideaThe job times are imo not really bad, since they are changed so active people get a higher reward. Which is why I would like to propose a system where you have at least two possibilities: Long and short jobs. For example:
I go shining shoes for a short time, let's say 2 mins. Job reward is 2 xp. If I do it a long time, 200 mins, you would think I get 100 times the reward, but instead I just get 125-150. So my reward is lower when I set a longer job, where my reward is higher if i'm more active and set a short duration.
This way you reward activity, without kicking busy players ass. Some people just can't login every 2 hours. That's a shame, but with max time of 2 minutes they are nearly forced to do a select group of jobs which take more than an hour or quit the game.