As an estimate on time, my players just finished the 4th session (4 hours a piece), and they just arrived at cragmaw castle. It depends on how long your sessions last, but I'd suggest you put a time constraint on them (which will inevitably be ignored) and start to look for a wrap up point (long rest, back to town, etc.) that will help end the day. Sometimes its unavoidable, but it helps us to have the timeline laid out. "Thursday from 5:30 - 8:30" (Note, that we've never actually wrapped up prior to 9).
My players DID actually climb up the chimney stack. Had to pull the rogue up, as she can't climb worth a darn.. but they used rope for that. They consumed all the mage's spells on the battle (sleep helped a lot), and took out the bugbear encounter right off. It was their first fight in the hideout actually, as the dwarf tamed the wolves using rations. It made going through the base backwards really easy for them (the traps don't spring), but I think that is a good reward for going up the chimney rather than the main route. Kuddos to them.
If they DID just leave, you could probably have a lot of people in town worried about Sildar. "What do you mean he's missing! Did the goblins take him? There have been a lot of goblins recently". Even if he's dead by the time they get back to the cave, at least they explore the whole thing.
Regarding maps, my players REFUSE theater of the mind. I'm unhappy with that decision, but the game is a cooperative effort and some compromises need to be made. In this case I've done what others have suggested and said "This is what you see, draw it if you wish". Also, I've printed out the artists work and cut them out into sections. They are too small to act as tiles, but they give people scale to draw on.
On a funny note, when you get to part 3 you might have to remind them about overland travel. The PC's bought 30 days of rations to go from Phandalin to Cragmaw Castle, then to Neverwinter. It took my players 6 in game days to get to Cragmaw castle from Phandalin. After the 3rd random encounter (feel free to read my Stirge posting) the rogue/map holder says: "Oh crap. I was reading the map wrong. Its not one day per hex is it..." Apparently they circled the castle about 2 times, and have plenty of food now.