There is no hard-and-fast or set-in-stone time frame for this; the trustee's obligation is only to act in a "reasonable" fashion. What is reasonable depends entirely on the facts and context: a down market may justify holding the property, until the market improves; necessary repairs or rennovations so the property will sell faster and/or for more may justify holding it; if there are any liens or claims against it which must be cleared up, that would justify delaying any sale; etc.