My question is: Can you use acrobatics to gain flank as part of a counterattack? And another question now that i have your attention… Once you flank someone, for how long are you considered to be in that position?

 Yes you can and only for one active action

 but if you go before your target, he will have -30 or -90 if he tries to hit you when his turn comes right???

 As stated in the rulebook on page 87 one may use Acrobatics when you have the right for a counter-attack or when it is your turn.

You simply make an acrobatics check against the targets attack or acrobatics, if you succeed with 100 you place yourself at the enemies flank, while 150 nets you a rear attack. These conditions will give the defender -30/-80 defense respectively. 

Juanzarabozo said:

Not really, as you've already gotten your advantage, when you attacked, and he's not surprised by the maneuver anymore. The opponent now just turns to face you. The only way to maintain the Flanked or Back advantages is continually making the checks for your (counter)attacks, or to get enough buddies around him to apply the modifiers, more than 3 (with 4th and 5th getting Flanked advantage) for a human sized opponent for flanking, and having a total of 5 other buddies (making you the 6th person) to get the Back attack advantage. These are called out on page 83. I interpret this to mean you can have up to 6 people attacking a human sized opponent at a time, exactly the number that can surround someone on a hex grid map.

Also, my interpretation, so if you wish to rule differently, feel free, but that is my reading.