Is there any way to preserve oil painted canvas without framing
As has been mentioned in comments, oil paint takes forever to cure and the process can't be hastened. If you need to store it without risk of damage, your only options are hanging, a shelf rack or fixed in a crate/box. As for how long they can be kept unframed, the answer is forever (or as long as the painting exists). Framing does not alter canvas tension or add to frame stability. The frame is an aesthetic addition that can serve as a buffer between the painting surface and large flat surface while stored. Most risks of damage still apply to a framed piece.