As far as I'm familiar with it, the Russians quiet pioniered that IRST tech. Today, the Flankers IRST -OEPS-27 I think- is said to have a detection range of 40-100km, depending on target aspect and signature.
Some F-4 versions actually had a FLIR before that, but these were later abondoned and not pursued on later aircraft. Probably because of the rather poor performance back then.
The F-14 had a TV camara on board to ID targets I think, but were not generally used to search for targets. Only now on aircraft like Rafalé, EF and the F-18 E/F blockII are long range IR searching and tracking systems introduced to western aircraft. Also with detection ranges of up to 100km. They are able to detect "hotter dots" before beackground. ID is only possible at much shorter distances though, unless you use IFF.
Those western FLIRs were more to get images of the ground ahead, but not dedicated air-search sensors until recently.
In your tank, I guess that detection ranges of several miles were probably enough (how far could you actually see?), but with the speeds involed in air-combat you need of course greater ranges.
But then again if you can reliably search for other aircraft at several tens of miles out, you can look for them without giving away your presence through active radiation.