240 dedicated circuit to subpanel
The use of flex in general is usually for short extensions called whips.  From a junction box to a motor per se.  When ran in that manner I believe the Code says "no longer than necessary" but I will need to double check.  

In your case it's not the same,  plus it will be under a deck so I think you will be fine.  Just FYI, it will probably be easier to run the wires through PVC conduit instead of flex as flex will bow and twist on long runs.  

Tip: Run conduit then a short piece of flex at the end.  Inside the house transition over to NM-B with a small 6x6 junction box

Addendum:

Code requires nonmetalic flex to be supported at intervals of 3 ft either by straps or wood members and within 12" of the termination points if it is not fished or if the length is &lt;=6ft,

For example under a deck that is accessible would require straps. Through a wall or under a deck not accessible would be ok to not strap.  But as recommend, PVC conduit would be a better choice for longer runs.