Help please - how long does it take a racehorse to run 1 length ?

0.2 seconds
1 second = 5 lengths

Many thanks

0.2's a decent rule of thumb. Actually, 0.22 is probably closer!  (If we're being pedantic, it does depend on how long the race is. You can turn the full race distance of the winner into lengths, and then use that to calculate a more accurate time for the losing horses.  But as a quick jotting, 0.2 is fine...)

NH is slower,its only .25 second per length.

Well precisely. The longer the race, the higher the second per length will be. US races don't exactly cover a vast range of distances. (1m 2f a 'marathon'???)   But if you're looking at a 2.5m or 3m NH race, using .2 per length will obviously give problems..

in mid-summer on a fast surface we used to reckon at about EIGHTEEN YARDS per second, I recall.

5 F fast track
is going to be a hell of a difference to
3M N.H heavy @ Cheltenham