So much of the game is opened up once you hit rank 50 so it should be something you work towards as soon as you can. Double XP boosts from the store or class packs on the xbox store will make this quicker, but isn't needed. Story missions and side quests are your best source of XP. Going through the story and cleaning up side quests as they appear will make this go pretty fast.
Make sure to change classes along the way as well to level them up. They level faster than character level. The story will provide you with enough commendations (earned through pursuits and contracts mostly) to buy another class for free, so it's a good idea to progress the story before you reach rank 25 on your first class so you can get to work on the next one sooner. You can buy the classes at the commendation quartermaster at The Crater.