How often is William Riker bluffing?

Riker bluffs in poker at least twice on-screen: once in 2x09 "The Measure of a Man", wherein he introduces the concept to Data, and once(-ish) in 5x18 "Cause and Effect". (The poker game in question was repeated several times due to time-travel shenanigans.) In both cases, his bluffing was relevant to the plot.

In most other cases the outcome of the actual hands doesn't seem to get much attention, so it's not clear how consistently he bluffed. He could play plenty of hands straight, and we simply don't see them because they're not interesting, sort of how we never see the hundreds of times the Enterprise pulled up by a planet and nobody tried to shoot, abduct, or swindle them.

Finally, from a poker-playing perspective, bluffing all the time doesn't make you good at bluffing. Any fool can do that. A skilled player should know when it's not prudent to bluff - whether because the other players are too confident in their own hands, or because they're starting to cotton on.

In addition to 2x09 "The Measure of a Man" and 5x18 "Cause and Effect" as mentioned already, Riker also bluffs in 3x26 "The Best of Both Worlds: Part 1".