Anything up to the 1mb limit. OP_RETURN doesn't give you the discount that inscriptions love to use, but it can still use up the whole block making no room for standard transactions.

So two benefits:

1. Initial block download isn't slow because of it (inscriptions made it so slow)

2. Blocks don't get filled upto 4mb because of it.

But that's IF people choose to use it instead of the signature based inscriptions.