Looks like you can play for free, no limits, from Friday at midnight (PST) to Monday at 10am (PST), and you can start the preload right now.

Korny just showed that you can bring a brand new character to 345 in about 6 hours. The faction rally's running this week, and you can get a character to level 50 in about 10 straight hours of play (you can cut that down to 5-7 if you spread it out over the week, but you've got 4 days here, plus you're starting from scratch, so it seems safe to assume 10 hours).

Do nothing but play Destiny this weekend, and you could have a Masterworked exotic on a platform you haven't ever used before! ;)