Typical escrow periods range from 30-45 days, however every facet of an offer is negotiable. In order to make an offer appealing, buyers who have flexibility to give plenty of time for the seller to identify their next home are in a stronger position.
A seller can ask to rent back for any length of time (or have the flexibility to do so, if needed). The rent is negotiable, from market value or even much less-- particularly in a heated multiple offer situation. Buyers who can be flexible and work with a seller's needs go to the front of the line.
This option offers the most protection for a seller, as the sale of their home cannot go through until the next step has been identified (and either put into escrow, or successfully closed). Of the three, this can be the least appealing for buyers as there's no definite end date. It's hard to plan a move when you don't know if things will fall into place in days, weeks or months. As a result, the pool of buyers may be smaller if this option is chosen.
This option can be appealing for a number of reasons. For someone who is moving to a place they are not familiar with, there's no better way to really get to know the area before a final decision is made on where to lay their roots. On the flip side, moving can be a pain, and having to do it twice isn't for everyone.