Medieval bows could not be kept strung continously for long periods of time because it would warp the wood (what's called string follow) and could weaken the wood if the bow suffers changes in temperature and humidity resulting in the bow breaking when fully drawn.
It's my general assumption that if you have weapon proficiency in a weapon, that you know how to take care of it and do so. I generally assume without being told that after battles and when breaking or making camp, that everyone in the party spends several minutes taking care of their goods. Likewise, a fighter wearing platemail is going to need about as much time oiling cleaning, and preparing his equipment as the Wizard needs to prepare spells. In the interest of game play, I don't make the PC's go through these tedious motions - though I make a point of mentioning it from time to time to remind them that things like this (and urinating and defecating) are actually going on.
As a DM, I very rarely I might take advantage of such situations to have the player's bow currently be unstrung, the fighter out of armor, and what not - but generally I would consider too much of that to be abusive on my part. I will definately do so if the players seem to be getting twinkish and insisting that they sleep in plate mail, always have thier bows cocked, never take breaks when marching, etc. Generally speaking, I run a pretty gritty campaign, and my players haven't plowing around in full plate mail anyway (having learned what a balance and swim checks are for much earlier on). If the PC's were planning long campaigns into the wilderness, I would expect them to plan ahead though by maybe having spare bows and spare bowstings on hand (or face the consequences).
I also make the assumption that magical equipment is made so remarkably as to be relatively immune to normal wear and tear, and so a magical bow can at least in the short term be considered above such worries (whether it would survive 50 years strung up on the wall of a ruin in an temperate swamp is a different matter).