It differs per auction what the payment terms are. Generally speaking a deposit and/or the transfer tax has to be paid within a few days (often no later than three) after the approval of the winning bid by the seller. The entire purchase price (including all costs) has to be paid within a few weeks (usually six), in accordance with the specific auction conditions.

You can find the exact payment terms in the specific auction conditions, sometimes it is also mentioned in the summary of the auction costs.