as time permits; whenever one has time; as long as time allows
A set phrase used to indicate that an action will be done to the extent that time is available, often in formal or written contexts.
Let's discuss this issue as long as time permits.
I want to enjoy sightseeing as much as time allows.
「できるだけ」 is a more general expression meaning 'as much as possible', while 「時間の許す限り」 specifically refers to the limit of available time.
A fixed phrase composed of 時間 (time), の (possessive particle), 許す (to permit), and 限り (limit), literally 'to the extent that time permits'.