On the planet Hoth, when Luke gets attacked by the Wampa and Han goes to look for him, we have this snippet of dialog (emphasis mine):
DECK OFFICER: Sir, Commander Skywalker hasn't come in through the south entrance. He might have forgotten to check in.
HAN: Not likely. Are the speeders ready?
DECK OFFICER: Not yet. We're having some trouble adapting them to the cold.
HAN: Then we'll have to go out on Tauntauns.
DECK OFFICER: Sir, the temperature's dropping too rapidly.
HAN: That's right. And my friend's out in it.
Given that the speeders are apparently ready when the Imperials attack a few days later, this leads me to believe that it took a week or less to finish adapting the vehicles to the extreme temperatures on Hoth. One would think that working vehicles should be a priority, so it brings up a question that AFAIK hasn't been addressed in the EU:
How long had the Rebels been on Hoth before the events seen on-screen in Episode V?