Depends on your definition of reasonable time but going to D1 after 6pm
Here is a rundown if you hit Shun Tak Center after work at 6pm. In theory, you should be able to get back before 1am depending on the ferry timetable and if D1 have rooms for your session if they are busy but seems quite rush. I usually sleep on the ferry rides to offset from the lack of sleep if coming back during the early hours of the morning
Buy package at Ferry Terminal - 10mins
Ferry ride & immigration (including waiting for ferry) -1hr 30mins
Walk to D1 from Macau ferry pier - 10mins
Pick a girl - 10mins
Session - 90mins (depending if its a weekday or weekend - weekend sessions are shorter by 15mins)
Walk back to ferry terminal to get ticket - 10mins
Ferry ride & immigration - 1hr 30mins
Total time: 5hrs or so if no hiccups