I have been playing SW:TOR for a couple of months now. So let me try to answer your questions.
First of all the game is a story based MMORPG. There are 8 different story lines to explore (8 different character classes) and around 15 different planets (shared content). The stories are good but not exactly on the caliber of a single player game, like lets say SW: KOTOR 1.
If you are playing 4-5 hours a day playing through the story line of a single class will take you around 1-2 months. At which point you will reach the level cap. At level cap (L 55 currently) the story takes a back seat and the game becomes more of a MMO, with raids and grinding and such. This part of the game is labeled as "elder game".
The game is free-to-try not free to play. You can create a character and you won't notice any restrictions until you are around L30-L40. After that the restrictions like the money cap starts hurting. It becomes almost too much trouble at elder game. Still up to that point the game is completely free and after that you can decide for yourself if you want to subscribe or not.
The game overall is quite good, it has its ups (good story, replayibility value, balanced combat system) and downs (bugs, f2p restrictions, not being a single player game, did I mention bugs? like Fallout 1 kind of bugs?) like every other game. If you decide to play a small reminder, the game character classes is quite balanced. No single class is much stronger than other, which means no all conquering jedi/sith. Pick a MMO role (dps/heal/tank) and choose a class accordingly.