If you are only after these variables you might as well use part2 only. assuming based on the filename you gave that you used something like the argument ilevels = c(0,30,100,400,8000).
if so, you can get the time spent in minutes in columns in part2_daysummary
[30,100)_ENMO_mg_0-24hr
[100,400)_ENMO_mg_0-24hr
[400,8e+03)_ENMO_mg_0-24hr
or part2_summary for average over all valid days
AD_[30,100)_ENMO_mg_0-24hr
AD_[100,400)_ENMO_mg_0-24hr
AD_[400,8e+03)_ENMO_mg_0-24hr
The above assumes that you have done all the necessary QC checks on the data beyond the GGIR checks.
Hope that helps,
Ben