If you never bluff, players will eventually figure that out, and you will suffer as they just fold when you make large bets, and the other times they will just endlessly bluff you out of pots you could, and should, be winning.
Conversely, if you always bluff, players will eventually figure that out too, and you will suffer as they just wait for their good value hands to win most of the pots from you.
So from this mental exercise we should realize and understand that we MUST have a strategy which is somewhere in the middle of these two extremes. How often should we bluff? Well that depends... it depends on how the other players at our table are playing. Are they the "never bluff" types? Then bluff away at them!! Or maybe they are the "always bluff" types, then value-town them all night long!! Usually we'll have a mix of both types, along with the more "pro" type of players who mix up their bluffs with their value hands very well and become reasonable impossible to read (the common phrase nowadays for this approach is called GTO or Game Theory Optimal, a statistical and even scientific approach to bluffing and to bet-sizing). And this is where poker becomes a tough game, we have to adopt different strategies against different players, its almost like doing mental juggling!! Good luck and have fun at the tables!!