MGTOC MGTOD Options, page-119

On the terms and conditions.

(b) The attaching options are exercisable at $0.05 each at any time up to 5.00pm (WST) on or before 31 May 2021

Which sounds like you could leave it to the last minute. Perhaps its worth emailing MGT to seek absolute clarification. It will take up to 10 days for them to convert as well.
Also depends how you are funding them as well, if you are using cash it doesn't matter to much when you convert, if you are selling heads then you will have to allow time for the money to clear through accounts.

I have started converting now. I'm just going to recycle the same cash through the process a couple of times then I'm going to start taking advantage of the arbitrage that's available.