You can continue using PA 5 (or PA 5.1) indefinitely in order to typeset any projects which have remained in Paratext 7, and have not migrated to a newer version of Paratext. Please keep in mind that the Send/Receive servers for Paratext 7 have been shut down and therefore getting project updates over the Internet is no longer possible. If any projects remaining in Paratext 7 are migrated to Paratext 8 you will need to begin using PA 6 or newer to typeset the project.