Yes - we'll always have a free plan, and yes, it does have a compute-time limit of 15 hours / month. All the details for the free plan are covered here (https://rstudio.cloud/plans/free).
Thanks for your understanding re: pricing - and we're definitely aware that for many universities, budgets are very tight, particularly at this time. We'll continue to do what we can to make Cloud as affordable as possible for instructional use - and I hope that you're able to find a way to make it work for you and your students.