I too have 10' but it seems excessive. For example, a 2' in diameter tree (which is wider than shoulder width of most men, you genetic freaks out there are the exception of course
) is 6' 3" in circumference which leaves a touch under 4' of tether length dangling by your knees give or take. Thinking of cutting mine down to 8' as trees I use are 12" to 18"
Both tether and lineman are 11mm static, I use samson assault line. Some use smaller diameter but I rather err on the side of caution with my life saving equipment.