In addition to, what has already been said, a phrase like "when should I bluff" is quite vaque, because there are 4 streets in poker including preflop. But its only on the river, where the situation is black and white in the sense, that either we have the best hand, or we dont.
When we bet any of the earlier streets, it can get better hands to fold (bluffing), but it can also get worse hands to fold (
equity denial) or have other advantages like building the pot and maintaining initiative.
So rather than mixing it all into one big bucket, I feel, its better to look at each street separately and ask a question like "when should I bluff the river". Or maybe even more broad "when should I bet the river". And on the river we can basically devide our hands into 3 different categories:
1) Hands that are strong enough to bet for value
2) In between hands that are mandatory checks
3) Hands with so little showdown value, that they are optional bluffs
Valuebetting basically mean, we think, we can bet and have the best hand more than half the time, when we are called. How strong a hand, we need to bet for value, depend on sizing, the situation and opponent tendencies.
In between hands are hands, that can not bet for value, but if the river goes check-check they will win at showdown at least some percentage of the time. These hands HAVE to check, because betting serves no purpose. This is a mistake, which is surpricingly common, where for instance you see someone make a big river bet with third pair without really knowing why.
Finally there are hands, that are bluffing candidates, because if we check, we will hardly ever win. This does not mean, we have to bluff, but we should at least consider it. And if we always bluff with the very bottom of our range, like maybe we have 43 and missed a draw, thats never going to be a big mistake. If we want to fine tune our bluffing a bit more, then questions to ask ourselfes are:
1) Can I tell a credible story meaning, are there any strong hands, I would play this way? If not there is a risk, the opponent might sniff this out and call us light, and its usually better to just give up and not bluff.
2) What kind of hands do I think, the opponent arrived to the river with, that he will now fold? Did the river change the texture (scare card), or does the opponents range contain many busted draws, that still beat my busted draw? In the latter case maybe a relatively small bet is efficient, since we are only trying to get hands like K high, Q high, J high etc. to fold.
3) Has my opponent shown strength or weakness in the hand? If the opponent raised preflop and made big bets on the flop, turn and river, its probably better to just let him win the pot rather than raise and expect him to make some huge fold with top two pair or whatever.
4) Do I have relevant blockers? I put this at the bottom of the list, because I think, it sometimes get to much attention. But if there are obvious strong hands on the board like 3 cards to a straight or flush, it is nice to block those, so thats its less likely, our opponent have these very strong hand, that we never expect to fold.