Right now I play NL10 SH and generally I try to find good spots to bluff. but of course there are still many fishes and stations at NL10 and I find bluffing causes very high variance.
At which stakes must we employ such bluffs to have a profitable wr?
Don't bluff fishes and stations 
Generally a low Fold to Flop Cbet and high Fold to Turn Cbet is a good time to 2barrel lighter.
River bluffs are pretty hard to get right, because often you get some wtf crying calls even from non stations because there's often so much money in the pot. The thing with river bluffs in my opinion is to only do them when your opponents range is very narrow and a disaster card for their range hits the river, and you can conceivably have it in your range. For bluffing regs, Look at your own game for spots where cards hit that make you wanna puke. Because when the situation is reversed, these are the spots to bluff back at your opponents.
accidentally quoted myself instead of editing post 
I can recall a hand the other day where i made an unusual bluff which wouldn't occur often.
Fishy station called both flop and turn on a J865 board with flushdraw. River was a blank 2 off suit and i bet something like 550 into a 900 pot with K10. Basically it was a disaster if i was called but i believe even more so if i check and he turns over an ace-high flushdraw.
The more you play, the better you become at reading boards and noticing player tendencies. People will say don't bluff stations as they will call with lots of hands and never fold but that in itself is very exploitable and not just because you can get more value when you have it.
You ask which stakes you should bluff more but what you should be asking is what players and in what situation should you be bluffing. As a rule it is usually the tighter the player, the more you should bluff but that goes alongside how coordinated the board is so practice on reading boards/players and in what situations would a bluff be more profitable than giving up.
I suck at bluffing but here is what I think:
1. Bluffing is not about limits but about opponents
2. On any limit it is stupid to try and get people of real hands - TP+ - they want to go to SD with them - who never called river with overpair in 400bb pot?
3. Bluffing is about making villains fold weak hand that is better than yours - middle pair, A high etc.
So what you need to do is to know who you play against. Wanna fire 3 streets - find a guy that c/c flop and turn with TT on K726r board and bet river vs him. See what he does there and adjust. Wanna bluff raise turn? Find someone that fire 2d barrel all the time and raise him.
Keep in mind that when you start bluffing more you will get called eventually. And that could be frustrating. It's one thing to reset mindset after unsuccesfull flop C-bet it's another to reset it after you spew your stack away. So even though the math is the same on the river - you bet 2/3 pot + they fold more than 40% = profit - the variance is much higher and if you have troubles with that it might be a problem later and cause bad play.
Finally one of the hardest things with bluffing is to get away from the "I wanna bluff now" bullshit and do it when there is actually profitable opportunity determined by stats, previous action of villain etc.
