at what intervals i should change heading so that my RDMI always points at 90 degrees.
As said above, you're not trying to keep the needle at 90º. All good tips, but personally I find chasing the VOR needle too much like hard work on a windy day, it needs to look sensible, but working with your DME distance and rate of closure pretty much corrects for wind and is for me much easier to assess.
I intercept the arc, turn, fly straight and see what happens. If you got your heading right you'll drift 0.2 DME inside the arc and then 0.2 DME outside the arc. When it reaches that point turn and fly straight to repeat. If you get to 0.3 DME inside or outside the arc, correct the heading. If you have a functioning FMC on the 737, then the wind arrow on the EHSI/ND removes the guesswork.
Personally speaking, flying a tighter arc diverts too much attention from other activities like checking FMAs, descending, setting flaps, speeds, monitoring traffic, etc. Flying looser and you run the risk of drifting a 1nm off the arc too easily.