You can respond when it is convenient for you. We think the quicker you respond the better.
A business with a receptionist will likely be able to respond either immediately or within minutes. If you are a single person running your business, you might be unavailable or are tied up helping other customers, and a response of even a few hours is okay. When you are tied up helping other customers, texting will be easier to go respond to than voicemails that were missed during that time.
The speed at which you choose to respond to texts is up to you. It can vary greatly from on business type to another or from one situation to the next. In general, the faster the faster the response, the better.