Is there typically a limitation on length from the control box to the
mat in a heated floor installation?

I'm relaying a bathroom floor soon and want to take the opportunity to
lay an underfloor heating mat, even though I may well not get round to
wiring it in for some time. The route from the void in which I'd stash
the spare connection, to where I might eventually fit the control box,
is relatively circuitous - maybe 10 metres. If I was fitting the whole
thing in one go I'd probably bite the bullet and chase the wall for a 2
metre route, but I don't want that disruption now if I'm not fitting the
controller yet.

I don't know how these systems generally go together - is it simply an
arbitrary length of T&E from the box to a short tail on the mat, or is
there a fixed length of special cable that Thou Shalt Not extend or modify?

Cheers,

Pete