I am considering electric u/f heating.How long should I expect it to heat
a
conservatory 10m2.

Electric underfloor heating can be very nice in a conservatory, particularly
with a tiled or stone floor. However, it should not be the only form of
heating for several reasons.

1. It is extremely expensive to run.
2. A conservatory is so energy inefficient that the floor temperature
required would be too large.
3. It is very slow to heat up the room from cold, meaning it has to be left
on for long periods when it isn't being used.

Instead, assuming you have wet gas/oil central heating, add an additional
zone with a fan convector. This will be much cheaper to run and will heat
the room up rapidly, which is particularly useful as you can leave the room
cold except when using it. Put it on its own zone with a programmable
thermostat, so it is only warm when you need it, with independent boost and
timing controls.

If you like a warm floor, have the underfloor heating as an additional
appliance, preferably using the lowest energy density you can find, or even
using wet underfloor rather than electric.

Christian.