Well, 10 minutes to warm up at 40 degrees is about right, it's the same
as my '84.  I doubt that the t-stat is bad, 12 o'clock is okay for the
needle.  If it was stuck open it would probably never get that high at
40, there are over four gallons of coolant in there to heat up!

Start checking the heaters, the plumbing and valves in particular.  Most
heat will come out of the rear heater since it is closest to the engine,
and if it has been bypassed instead of plugged off you won't get as much
hot water up to the front.

The flow to the heaters is not affected by the thermostat, it comes out
of the right head and comes back directly into the intake side of the
water pump, bypassing the t-stat, so you should start getting some warm
air after 3 or 4 minutes of driving.

Good luck!
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT