Unfortunately, expired entries will only be deleted if you attempt to access the transient again. If you don’t access the transient then, even though it’s expired, WordPress will not remove it. This is a known “issue” and is due to be corrected at some point in the WordPress core code.
-Copied from: Artiss Transient Cleaner
Found this plugin, can help you:
Additionally, can modify the time of the deletion using the process mentioned HERE