How much time would it take for a plesiosaurus to get enough breath at the surface?

In the MM (p.80) The Plesiosaurus can hold its breath for 1 hour.

However, it doesn’t stipulate how long it needs to take enough breath to submerge again.

What would be an adequate time to allow it to “recharge” it’s breath, maybe 1 or 2 rounds, or longer?