According to a senior member of the bipartisan British Commons Defence
Select Committee (the UK equivalent of our House Armed Services
Committee), shortly after the group returned from personally surveying
the situation in Iraq for themselves:

10 to 15 more years.

Shocking, yes.

Of course, I pointed out
almost a year ago (thanks to a heads-up from USAF Lt. Col (Ret.) Karen
Kwiatkowski) that the U.S. was expecting to remain in Iraq (to use
former U.S. interim administrator Jay Garner’s own words) for "decades."

Also shocking: so few in the public debate seems to acknowledge this was always the idea.