One of the characters I leveled from 43 to 50 in about 7 or 8 hours. I did this by:
1) using the Double XP
2) combined with the 25% bonus buff and
3) I played through the class story line and the planet story main mission line only.
I found that by doing this, by the time I reached Corellia, I was level 50. By the time I finished the planet I was on at level 43, Belsavis, I was way ahead of the level requirements for all missions, making it very easy gameplay.
By sticking to just the class story line and planet story main missions, it was almost like being involved in a movie. Gone were the distracting side missions that I've already done four times by now on other characters. I generally find that leveling characters, especially alts, can be a little bit confusing when it comes to stories, especially if you have a gap of weeks between major chapters. Throw in one or two other alts stories and it can be quite confusing.
With my second character, my Comanndo, I used a different tactic. I was invited to join a group in my guild that was power-grinding on Ilum. A full raid of 30 players were all leveling at lightening speeds. So I took my level 43 Commando out there and within 2 hours he was level 50.
This coming weekend I have a level 14 Inquisitor to level. In preparation I have maxed out warzone experience and class mission experience bonuses in my legacy for this character. I also have boosters in the character's inventory. I intend on playing through her class story line and PvP only, skipping all other side missions. This will give me more than enough experience and credits to have an enjoyable leveling experience, whilst still achieving my goal of getting to level 50 by the weekend's end.
In my quest to provide detailed companion information, by the end of this Double XP weekend I will have all eight classes with completed class story lines - and also all companion story lines completed to maximum affection.