I don't know about 30 min......but for the guys that practice 3+ hours... (not trying to be noisey buuutttt) Are you more of the professional bassist?? Do you have kids, 8 - 5 job??
Everyone situation is different, just trying to find out more.
Are you asking for those who practice more than 3 hours a day, or more than 3 a week.
My practice time includes, Friday night rehearsal with the praise team. which is minimum 3 hours.
I typically don't practice Sunday or Wednesday Nights, I play in both those services.
Which leaves me, Monday, Tuesday, Thursday and Saturdays open. Saturdays are typically designated for family time.
SO now I have Monday Tuesday and Thursday. I'm a minister so I have to prepare messages, spend time with 4 kids, wife, and get my personal time with the Lord.
So as you can see... Practice can't be top priority for me. I get 45 min. to an hour on those last three nights... Sometimes more, sometimes less.