Depends on your rental agreement. You can book the vans for as many days as you want or need. I'd visit uhaul.com to investigate your options or give a call.
Rentals are based on availability. The weekends and end of month are usually busy and six or eight hours is the maximum
Usually a 24 hour rental, unless it's the first of the month
No they give you the option when reserving to keep for hours or days
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