How far ahead you can go depends on your plan: the Offer closing date field in the Budget step of the demand wizard only lets you pick dates up to the last day your plan allows — anything beyond that can't be selected.
To see the exact number of months, open Settings › Usage and look at the Offer deadline row under Plan limits. That row is a buyer limit, so it shows the cap that applies to you as the person opening the demand.
The helper text right below the date field repeats the current cap, so you never have to guess. If you type or paste a date past the cap, it isn't accepted: a warning appears under the field with an Upgrade to Pro button next to it, and you can't move on to the next step. Moving to a higher plan widens the cap when you need a longer bidding window.
There's a rule at the other end too: the earliest date you can pick is tomorrow — same-day closing isn't allowed.
Also asked as: What's the maximum offer deadline? · Is there a limit on how far out the closing date can be? · Why can't I close my demand 6 months from now? · Can I give suppliers a longer bidding window?