It almost totally depends upon how experienced of a player you are and how grooved your strokes are.
Some folks can get their strokes and timing back after only 5 or 10 rallies after being out for over a month with an injury. Other folks can struggle with it for over a month.
A couple quick tips:
1) Pay attention to what you eat. Choose the healthy choice a majority of times ;-)
2) Do other things to stay in shape where you can - go for a run, hit the gym, or just do sit-ups, core work, lunges, etc if you don't have equipment.
There are things you can do to stay in shape pretty much wherever you go.
I've done sit-ups, crunches, and core work while on Safari in Africa. Some environments aren't well suited to going for a run - unless you want to be a predator's meal ;-)
Have Fun! K_I