Welcome back, Mark.  :)

I have to join in to Viethson's post but I wouldn't say that it drifts too fast but faster than on the Concorde for example. I usually start my DME update at the gate (if I get reception from a DME of course), so that I don't have to care much on the initial climb until level off.

Maybe they've tweak the behavior on the Conc as she really needs the accuracy there and travels a lot fast while developing a higher need for fast updates.

I'm not so much into the actual gauges and I also haven't really measured the loss if alignment, but my subjective feeling is the same as Viethson's, the gauges in our birds are more sensible.

Maybe some rw flyer can explain what actually leads to faster loss of alignment. I could image that a straight and level flight will hold the INS in shape while all turns and climbs lead to fast misalignment.
As you turn and climb out your SID, you may therefore have the highest need for an updated INS.