An average cycle in the washing machine lasts between 25 to 40 minutes. Expect each dryer cycle to last about the same length or a little longer, 30 to 40 minutes. If you simply can't wait the half hour and plan to abandon your station to run an errand, make a note of the time you started your load (most laundromat machines have timers) and plan your return to coincide with its completion. Even better, come back a few minutes early. This, according to (mostly unspoken) shared laundry-room etiquette, is important. Leaving your clothes unattended is considered rude because you're monopolizing machines.
Anecdotally, odds are pretty good that if you don't move your clothes, someone else will — they may give you a brief grace period, but any longer than 15 minutes and you can expect to find your clean load on top of the machine, in a dryer or on a folding table. (Or, on a particularly bad day, on the floor.) If you're a wash-and-go type, leave your basket on top of the machine.
It's not only that unattended clothes waiting for an owner wastes everyone's time; unwatched clothes are also a target for thieves. Laundromats are public spaces, and if you're not watching your clothes, someone else might be.