Was 5.25 hours before Droylsden-Ashton and Airport opened, back and forth on airport add 1:30, back and forth Ashton add 28 minutes so roughly 7-7.5 hours.
Actually I did it in 4 hours 51 minutes 5 seconds on 29th June 2013, which was before Droylsden-Ashton, the Airport line, and also Rochdale Town Centre. I have all the details on a Microsoft Word file. I say I visited every stop, but it was a Saturday and I passed straight through Woodlands Road twice without stopping so I may have failed on a technicality of course!
I have just consulted the Metrolink timetables on the TFGM journey planning website.
If tells me that it should be possible to visit every stop in 6h37m!
10:05 Rochdale Town centre to Shudehill 10:56
10:59 Shudehill to Bury 11:24
11:29 Bury - Market Street 11:56
*toilet break in Burger King perhaps*
12:09 Picc' Gardens to Ashton 12:42
12:47 Ashton - Eccles 13:51
*toilet break in Morrisons*
14:01 Eccles to Broadway 14:11
RUN
14:19 Media City to Cornbrook 14:30
14:36 Cornbrook to Alty 14:54
14:59 Alty to Trafford Bar 15:15
15:22 Trafford Bar to East Didsbury 15:39
15:49 East Didsbury - St.W Rd 16:00
16:10 St.W Rd - Airport 16:42**
**In reality the Airport line timetable is very challenging, I have been on the line 4 times and all journeys take ~3 minutes longer than timetabled, therefore I would not expect to arrive until 16:45 giving a total travel time of 6h40m.
.....and yes, you need to think about a quick toilet visit, believe me!