Quick answer: forever. As long as you monitor your water quality and do water changes as necessary a skimmer is not required.
A skimmer will remove organic material, which will reduce the need for water changes and help keep water quality stable, which will be more important when you switch to reef set-up.
My advice, I wouldn't be too worried about it while in FOWLR mode, but I would probably install one before I put any coral or other sensitive reef creatures in the tank. It can takes a few days for a skimmer to start working properly, so be prepared for a "break in" period.