How much time should I allow for math postdoc candidates to accept or decline my offer? For the first time in my life, I received a research grant that allows me to hire postdocs. The hiring procedure is very flexible, and I have complete control over the process, without any involvement from my university or the grant agency.

My question is - how much time is it customary to give to potential postdocs to decide if they accept my offer?

I am especially interested in feedback from pure mathematics.
Supposing this question is not hypothetical, and you are at one of these U.S. departments (assuming you need respect the agreement), you do not have too much choice in the matter. The departments at that page have agreed not to require responses to postdoctoral job offers before Monday, February 3, 2020.

Otherwise you could place it before January 24th (the deadline for decisions on the NSF postdoc), in hopes of snagging someone who might get a more attractive offer later, or afterwards, to avoid having someone else renege on your offer.

This all depends on where you are located, and what the status is of your potential candidates (the agreement above applies only to candidates within two years of their Ph. D.).