I've played in games where all you needed was time. You could do it on the road.
3.5 is more explicit that you need access to some sort of lab. They assume that you have access without really specifying it in the rules...probably just to prevent it from being something you do out in the wilds of adventureland.
My next game I plan on running will be a bit different, since feats are (in general) going to be something that you need to find specific trainers to provide, at least through 12th level or so. Basically from 2nd through 12th, you'll need to find trainers to teach you the things you want to use. I'm also moving heavily away from Prestige Classes, in favor of prestige feats that are simply more difficult to find the appropriate trainer.
This game will have a slower progression through levels, and a lot of game time passing (months to years)...allowing for roleplaying opportunities and the chances to find trainers along the way. It also means I can use trainers as another reward in the game. Essentially, feats can't be taken for granted...and if you want an interesting feat, or a prestige feat...you'll have to earn it in play.
In this world, actual crafters of permanent magic items are rare, don't advertise much, don't like interruptions, and don't like rocking the boat by expanding on the number of people who can do their jobs. Those few that don't craft for the nobility rarely craft for anyone but themselves. So, PC crafters will certainly be rarer in this world. =) OTOH, if someone does roleplay himself into a crafting role, they'll definitely have the game time to use it, if they can come up with the cash to feed it.
In my previous campaign, a typical storyline was 3-6 sessions, at the end of which the group was usually back in the main city/locale for the game. After an adventure the group would have downtime for the rogues to gather information, the fighters to be bored, and the spellcrafters to craft all sorts of new items to give me nightmares.
