MyChinaVisa has important information on permitted stay under the ten-year visa. It seems it will be sixty days for L and M visas. According to the FAQ there, you may be able to get a longer stay, but not on a ten-year visa, unless you qualify for an S2 or Q2 visa.
"All US passport holders can apply for a 10-year multiple entry visa in the following categories: tourist (L), business (M), and family visit (Q2 & S2). For 10-year multiple entry visas, the duration of stay (the number of days you can stay in China per entry) is 60 days for L & M visas; 90 days for S2 visa and 120 days for Q2 visa. If you want to stay for more than 60 days (L & M visas), 90 day (S2 visa) or 120 days (Q2 visa) per entry, you may be issued a 1- or 2-year multiple entry visa, not a 10-year visa."
Has anyone found out if USA passport holders are able to receive the 10 year M visa in HK? I will apply in early January if this is the case (my current visa expires end of December).