tipping is not a requirement, it is always most welcome.
When and how much is totally up to you.
If the treatment
was outstanding and it leaves you satisfied, by all
means tip anthing from 10% up. You should, however,
not feel obliged to tip - you should still receive a
good service even if you don't.
If you're a regular
and you make use the same facility quite often, it would
be nice to leave a tip regularly. When in doubt phone
the spa manager and aks what would be suitable. You
can leave the tip at the reception desk when you pay
your bill, just specify who it's for.