Time or distance for heat to start appearing from heater.

[Will]

I wish they had bleed nipples for this. But when I install the new thermostat I will do my best to bleed the system. One trick I tried was wind gaffa tape to a funnel to make a good fit on the filler top up as usual and fill the funnel so the water level is higher. Thus encouraging the bubbles to escape from a higher level. Also park the car on a slope. (But was it facing up or down?)

You probably want to park facing uphill - ie raise the front of the car, so the matrix is lowered slightly.

Run the car with the cap off the expansion tank from cold, let it rev a few times to get the water pump up to speed and try to burp the air out.

On some cars, it can be difficult to expel all of the air from the heater matrix without specialist techniques/equipment. You could remove one pipe from the matrix and flush through with a hose from the other side, then pinch/clamp the hose off before refitting. At least that way you’ll know that the matrix isn’t blocked as well