Generally speaking, you need to play 20 minutes with someone to see your opponent's table image. Getting good tells takes longer and the higher the level, the longer it takes.

My english is poor, so this is what i understood: I need to be playing 20 minutes on a table to know the style of game that is using my opponent? (a little wierd because if u have a good data base u dont need to wait nothing to know how is ur opponent :S )
The same happen on every stake of course u should have up to date ur data base

You also have to "represent" a hand. You can't just decide to bet big after the flop comes 7, 8, 2 and the turn comes a 3. What are you saying you have by betting here? Will it be believable? Are you saying you just paired the 3? If the turn had come an Ace, and you don't think your opponent has an Ace, that would be a good time to bet big "representing" the Ace. But don't just bet senselessly or he won't believe you and you'll be caught. You must create a believable story.

Totally agreed with your answer inthedsm with " you must create a believable story" but there you are assuming a bluff and im just saying that you have to see some things before do a bluff.. 
Imagine that situation: flop: 7,8,2,3 and you opponent has bad stats to bluff him and your image isnt the appropiate...are you going to bluff him? of course that no... or maybe you could have the appropiate image but the stats from your opponent arent appropiate.. in that case its ok maybe you can bluff him one time or two depending in the inteligence of them...
= by general the people what doesnt fold in a flop like: 7,8,2,33 is because they have bad stats to make them fold (VP$IP high) this mean only one thing: more probability to connect a pair and people with good stats to make them fold by general they have stats like -20 of VP$IP this mean only one thing: they too didn connect a pair

Its not just "create a believable story" you must "tell a story that villan understands and believe". Maybe, your story makes sense, but what if you are bluffing the fish?, he cant understand your story.

Of course you are going to bluff few and nothing with fish players, because like you said... they dont understand nothing... in other words they have bad stats to bluff them