Although I agree with Lampwick’s advice, half an hour a day isn’t as much as I’d like to focus fully on the PE task in hand. It would cause me to rush things and try to fit in as much as possible, thereby putting too much strain on my unit. I believe 45-60 minutes is ideal, with rest periods of course, to make the most of a routine.
I appreciate that for the majority of members here, time and privacy are of the essence, but the more advanced the PE technique, the more time should be devoted to performing it correctly. Given 30 minutes, I’d either combine manual stretches and jelqs, or try to fit 2 good 10 minute clamping sessions, with 5 minute rest/warmup inbetween - there is little time for anything else.