There's no need to spend more than 10-15 minutes in one spot if you are not catching fish after a few casts and a couple of presentation switches.
You are either:
a) Not getting your presentation down to the fish, so your effort is futile.
or
b) There's no fish there.
A Vedder old-timer averaging 50-60 fish per season told me this a couple of years ago, and I have finally started to listen.
Of course, I will now hear from the guy who said they got a steelhead after having fished the same spot for two hours without success.
Or maybe not, no one will admit they are fence posters publicly, will they?