LAN8720A - Reset Signal Rise Time and Fall Time Requirement
There is no hard requirement. The Reset signal needs to be clean. A Reset signal which is a monotonic signal with less than 1 ms rise time should be good.
More information regarding Reset signal timing requirements can be found in on the "Power-On nRST & Configuration Strap Timing" and "Power-On nRST & Configuration Strap Timing Values" tables of the LAN8720A datasheet.