Training? Even the world no.1 player like Novak can’t find a legit. free court (see Spain incident) to practice on right now so these ?’s r premature but at least the Swiss #1 is probably healing physically & mentally very soundly.
Most (at ATP tour level at least) will still be keeping fit, so I'd say no more than 6 weeks including 1 or 2 tournaments would be fine. The long rest period may even help some players who grind every week.