Short version: call them now and ask.
Long version: There’s no reason to avoid or delay asking about the terms of a possible contract, other than the fact that the exact terms may undecided. Most commonly, the rate of compensation and amount of leave may depend upon negotiation after they have determined how you will fit into their organization.
The probable length of a contract should be known in advance and it is appropriate to ask how long the contract will last before the first interview. It is in fact one of the most common pre-interview filters, both sides will almost certainly have a minimum length before they would consider the contract. Consider, that some contracts may be for as little as a day others may be as long as several years. Someone looking for one is unlikely to accept the other, and a company looking to offer one is unlikely to agree to offer the other.
Contract length should be at least ball-parked (2-4 days, 3-6 months, 1-2 years, permanent position) before you get to the final interview in order to avoid wasting everyone’s time.
For example an employer asking me to interview for a 1 month contract, would be wasting both our time’s — better to say no thanks at that point and hang up the phone. For someone else, that may be the ideal period, asking them to commit to more may be just as futile as asking me to take a 1 month contract.