About 2 months. I've just used an Astra Superior blade with great results, no problems whatsoever and using the same safety razor.
Are you using the Muhle R89 (new design) razor?
Yes I am using the new design of the Muhle razor(a.k.a. "new Muhle 2009 head").
I had great results with Iridium blades both in the R89 and R106 Muhle razors.
I remember I tried the Astra in the Merkur HD razor, but it was really rough..
My next blade that I will try in the Muhle razors is the Astra!
What razor are you using?