A320 VOR Approach
 

what is the proper time to press the T/FPA button during the approach,I usually do it when I fly heading to intercept the radial inbound,but some guys said you have to press it only when you are fully established on the radial others says you can do it fairly on earlier stage does any one have an idea because the sop doesn't say any thing about it.

How have you been cleared cleared to intercept the final approach track?

If in managed nav, then I can't see why you don't select TPA when you wish.

If cleared to intercept off a heading from ATC, then technically when you're established inbound, as intercept heading and track will differ by your drift angle, and track fpa will change your heading to maintain the selected track.

is there any reference that talk about that?

During intermediate approach when cleared to intercept the inbound course, either localiser only or VOR or NDB, Use TRK mode to align the aircraft. Ref is Airbus Industrie Instructor Support, page 55 Normal operations.

It makes sense to remain in HDG with a great big VOR displayed until the CDI is centered in my books, then once established, TRK mode. Remember these are guidelines by sense must prevail.

Personally I would fly it with ARC or NAV mode displayed with the VOR on the needles. You get better situational awareness that way.

On the intercept heading with just a few miles to run (ARC mode helps here) switch to TRK/FPA. The heading won't change much in a few miles despite flying track.