Can my customers see the cancellation deadline for a hotel room before they book, or do they only find that out afterwards?

Yes, the cancellation deadline normally comes through with the room rate and can be shown to your customer before they pay. Hotel suppliers return a policy alongside every rate, and that policy usually states the date and time after which a charge applies and how much that charge will be. Your portal can display this on the room listing, again on the checkout page and on the voucher, so the guest reads it while they are still choosing rather than discovering it later.

How clear it looks does depend on the supplier. Some send a tidy date and time that the portal can format neatly and even use for a free cancellation filter. Others send a plain sentence of text that has to be shown as it is written. And some rates are non refundable from the moment the booking is made, in which case the portal should say so rather than show a deadline. Deadlines are also usually set in the hotel's local time, which is worth spelling out for a guest booking from another country.

The honest limit is that the deadline belongs to the supplier and the hotel, not to you. You cannot extend it, and if your own agency terms add a service fee on top of a cancellation you need to state that separately in your own booking conditions, because it will not appear in the supplier's policy text. The policy is also tied to the exact rate that was quoted, so if a customer comes back days later and searches again, the rate and the deadline may both have changed.

It is worth making a test booking on your portal and checking that the policy wording appears in all three places, on search, at checkout and on the voucher, before you start selling.