Not particularly long for me, but with on and off practice. You don't need CLL to get sub-5; it's possible with LBL/Ortega. That's not to say you shouldn't learn it though, as it'll help later on.
About a week I think, but it really depends on how fast you are at 3x3. Learn Ortega, that will easily be enough to get you sub-5 if you're sub-20 on 3x3.
I'm sub 20 on 3x3 but I'm struggling to get sub 7 on 2x2. I guess I didn't practice enough. I've only done about 1000 solves on 2x2. I guess I need probably another 2000 solves to be sub 5.
Once you know full CLL you should definitely be sub 5. Make sure you're one-looking your solves, if you have to pause after FL then that's definitely a problem. Try doing first layer blindfolded more often. In fact, you don't even have to be able to one-look a solve to be sub 5 avg on 2x2, you might just be doing first layer wrong, check out this video by Chris Olson