I always just stretch my arms out as far as they go and that's how long my linee are. I never measured them but I'd guess 6 to 8 ft. They always seem to be long enough even with the hiking pole porch setup. That's always been my barometer...long enough to set up the porch.
I have 4 lines (of 2mm dyneema) permanently attached to my tarp (overhand knot on the end then larksheaded onto shockcord loops) which are each 2m (~6.5ft) long, which to date has been plenty long enough. I do carry extra cordage so if needs be I could always extend the lengths.
I start with a 10' piece and splice eyes on each end, so they prolly end up 9'6"-ish.
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Deep peace of the running wave to you.
Deep peace of the flowing air to you.
Deep peace of the quiet earth to you.
Deep peace of the shining stars to you.
Deep peace without end to you.
adapted from - ancient gaelic runes