Do clients buy a prostitute’s time, or specific sexual acts?
It depends on the prostitute and what the client chooses to negotiate for. Among hustlers, it is customary to negotiate for a specific act. The hustler will perform the agreed acts, provided the client does not take an unreasonable amount of time in the process. What is unreasonable is entirely subjective, and is resolved between the specific hustler and client.
Escorts, especially those in countries where prostitution is illegal or legally ambiguous, usually prefer not to discuss sexual acts in advance. Instead they agree to spend a set amount of time with the client. Because of their apprehension of discussing sex acts in advance, finding an escort that is willing to do the sex acts that a client seeks can be awkward and difficult.
Dancers are usually somewhere between the two extremes. Some dancers provide the best of both worlds, with a willingness to discuss sexual acts in advance and an agreement to stay for a certain length of time. Unfortunately others are the opposite. They not only refuse to discuss sexual acts in advance, but they are also not even willing to agree to stay for a given length of time.
Erotic masseurs and bodyworkers typically sell their services for a given length of time. It is generally assumed that their services will involve massage and masturbation, although there is some flexibility to this rule.