Backcounting
There is no point in backcounting a game if there is no way for you to jump in on the game when the count is favorable. If the tables are completely full, you will need to play elsewhere, or play after the before/after the casino is too busy. If the tables are nearly full (only one or two spots open), I will sometimes lower my standards in terms of what count I will wong in. If I would normally jump in with a 0.5% edge, I may go ahead and place a smaller bet if I have a 0.0% edge since I want to have the spot available if the count continues to go up. Another option might be to hire a spotter to actually sit and play at one of the full tables. Of course, the logistics of them leaving the game and getting you in the table to play their vacated spots can be a bit tricky and may be difficult to pull off smoothly more than once or twice in a single session.
Walking around and hawking tables does feel awkward at first, and in all likelyhood looks awkward to anyone who happens to notice. This is where a busy environment is helpful to you since you can more easily blend in with the crowds. You need to look like you're doing something besides backcounting. Watch a few spins of the roulette wheel, watch a couple hands on a carnival game, or just stand back and watch TV some of the time. If you can do this while backcounting a BJ table, even better.
For max amount of time, I'm not sure whether you're asking about how long you should stay in a single casino or if you're asking about when to decide to give up on a shoe.
In terms of your session length, it depends greatly on where you are playing and on the current conditions. Generally speaking, if it's a larger casino with several tables and pits, you can stay longer than you could if it were a small place with just a few tables in one pit. If it's crowded, you can go unnoticed longer than if it's not crowded. If your bets are big enough for them to take notice, you may only want to wong into a game once or twice before moving on. If your bets are comparable to everyone else and you just blend in, you can play longer.
In terms of when to give up on a shoe, if the Hi-Lo TC drops to -1 or worse, it's usually time to start looking for another shoe with a fresh shuffle.