Qulom, welcome to the forums. First thing to do: use our search function. A lot of the kinds of things you're asking about have been asked in the past, and answered at great length but many people. The site owner doesn't allow duplicative threads, so everybody's answers tend to be gathered. At a thought, try searching on these:
classes
recommendation
profession
spells
stats
attributes
I am not going to close your thread, though. Because everybody starting a new game has their way of looking at these matters, and their followup questions about party construction.
Then, get yourself a nice meal and a good, tall drink, because you're going to have quite a time reading dozens or more well thought-out posts.
As to how long BG1 should take...? Easy question: as long as you require. There isn't a deadline on enjoyment. And in case you weren't aware, in the BG series, unlike NWN, you can run multiple mods at the same time, all within the basic game. There are a number of fine ones for BG1 that add quests, create new party NPCs, enhance the dialog of existing party NPCs, and add in the BG2 engine (which means several enhancements, including map notes and highlighted objects on the ground). BG2 has many, many more, several dozen, in fact. It's best to start a separate thread for mod recommendations, but I would urge you to look
here, at the Pocket Plane Group, for some idea of what's available in the way of mods.
For myself, I would first suggest that before rolling, you install the official patch from Bioware for BG1. Then consider adding something called
EasyTutu. A number of BG1 mods require this, and it enhances the game. You can see what it adds from that page I linked to. One excellent mod to use after installing EasyTutu is
The BG1 NPC Project, which adds a lot more depth to the party NPCs you find, there.
After that, although you can use multiplayer to build an entire party yourself, I strongly recommend starting one character and picking up five more as they show up through the game. This really enhances the roleplaying aspects of BG1, and there are considerably more than 5 to grab. As for classes.
As for classes, my suggestion is to use a cleric or fighter. That's because magic, while important, is much less powerful in the game than in BG2, and in the early stages of gameplay, having an effective character with decent hit points is probably best. Just my take. In BG2, there's an excellent mod that gives clerics a lot more roleplaying flexibility, so that's something to look forward to if you plan to port your character across between the two titles.
As for stats, you do have the manual, don't you? That should explain what each stat basically does. Fighters obviously benefit from high strength and constitution, and dexterity lets you avoid incoming missles easier. Charisma helps get lower prices from merchants. Wisdom is of primary interest to clerics and druids, while intelligence helps mages and bards. Although bards and druids are fun to play from a roleplaying perspective, their classes are (in my opinion) severely hobbled in the BG series.