I can't remember the specific directions but it's definitely not every day. IIRC it was one sunny afternoon every 2 weeks. We have more laundry than an average, 4-person household so I try to do one afternoon a week. I will often put the balls out when I go to bed (in summer) and then get them at some point in the morning. Then they get regular morning sun. In winter, though, we can go a long time without a bright sunny day. I just give them sun when I can. Mine do not get smelly. I've thought about getting a second set just because we have grey winters and lots of laundry.
I am fanatical about keeping my washing machine aired out, though, too. The door is always open when not in use and I remove the laundry drawer for extra air circulation. And I use distilled vinegar in the fabric softener dispenser in every load to reduce mineral build-up. I use
Kleenite about twice a month.