checked the log book for a couple of dives, note HP 100 tanks so you can do the ratio
1--cold water + drysuit = 58 minutes, max depth 40ft and ascending steadily most of the dive, albeit slowly and at less than 20ft for quite a while
2--night dive, cold water + drysuit = 48 minutes, max depth 40 ft, ascending again, average depth overall prob 30 ft, actually ended dive due to chilled as opposed to lack of air
as the other comment said, I've never really stayed at 40ft to find out, but age has caught up to me wrs air consumption