The
pre-rework version says they last as long as MEW does, which last on the order of
20 minutes (again, pre-rework).
A quick look-over tells me that HLaM should state that its effect ends once the user leaves the ground (as demonstrated during the Hazou vs. Keiko fight) but it's possible to not do so (as demonstrated during the Team Gouketsu vs. Team 8 fight, round 2); Tunneling Excavation in
Chapter 162.1 was noted to have produced a
sagging tunnel with the implication that it would have collapsed given time (suggesting that the permanent tunnels could collapse if made in structurally unsound areas).
As for seals, Banshee Slayers should be within a
3cm radius (not 2cm).