The time they gave me recently for my guaranteed rate was 62 hours (that is probably because I did it on a Saturday) - from the time I started the process by signing in and 'sending' to a recipient. Then they allow a period of time for the sender to transfer money online from their bank account (or pay by other methods - which are more expensive) and if the money is not transferred within that time period, the transaction is deemed cancelled. You do not have to 'send' and pay at the same time, but obviously they will not actually send any money to a recipient until they have received the original currency from the sender.

This delay period can be used to advantage by checking rates and deciding if to continue (by transferring the cash to Transferwise for them to convert and send on to the recipient) OR to cancel (by the sender NOT transferring the cash to Transferwise within the specified time period - or, more politely, by using the 'cancel this transaction' choice on your account on their website).

They do say the rate is guaranteed and they are a sound company to deal with - but if the rate fell from say $49 to $40 I am not 100% certain what they would do! Some financial institutions do buy currencies in advance and this may be part of their business model.