It's 48 hours. You can be legally expected to work anything up to that without overtime pay, as a salaried employee.

And penalties for requiring employees to work over that, apply only when this is exceeded when it's averaged over a 17 week period.

But you're right, many fields pretty much bring up the 48-hour overtime exemption contract to you on day 1, many temp jobs, or things like catering/bar-work require you to sign one of these.

UK implementation of the EU Working Time Directive allows employees to waive their rights. Mot of Europe doesn't have that model.