There's no minimum number of hours set for the GVC flight test. We'd suggest around 4 hours of practice beforehand, which is what most candidates find is enough to go in confident rather than hoping.
Module 11 of the online course lists the specific manoeuvres the assessment covers. Practise those rather than just flying about, and you'll get far more out of the time.
Worth being clear on one thing, because the numbers look similar. This is a recommendation for the GVC practical. It's not the same as the A2 CofC requirement, which is a hard minimum of 4 hours of logged A2 training flights within the 90 days before you upload your logbook, reduced to 2 hours if you already hold a GVC or an RPC-L1, L2 or L3. That one is set out in Practical Flight Training and Flight Log Requirements.
If you're not sure which applies to you, ask us before you book anything.