The open rates for SMS messages are amongst the highest across any form of marketing. Recent reports found that compared to email marketing, which has an average open rate of 22%, the huge 98% open rate of SMS messages is even more impressive.
However, there is no average response rate that can be given with any exact certainty.
The success of SMS marketing varies and depends not only on the product/service being sold & the audience and their receptiveness, but the wording you use, the time of day you send the message and how you require people to respond to acknowledge interest.
It is not possible to track SMS opens like you can track email opens. But depending on the country / carrier where in you are sending SMS, you will be able to get visibility of the message delivery from SMS History object report.
If you wish, you can provide a link in the outgoing message like https://www.google.co.in/ and track those conversion - not the same as opens, obviously, but at least a way to measure engagement.