There are some people (especially members on the LSS committee) who underload in some years and therefore take longer; however depending on your non-uni commitments it is definitely possible to complete the double degree in 5 years.
I'll also note to those reading this that whilst the double degree is a 5.5 year workload, Monash structure the course so that students overload two units at some point or complete two summer units (generally arts units); resulting in the course being completed in 5 years.
The main difference re: time between the two paths is therefore that the Monash path can be completed in a minimum of 5 years; whilst the minimum is 6 years for Melbourne.
I mean talking about minimum times gets messy because if you
really wanna overload you can do the melbourne uni combo 4.5 years (2+2.5).
you can do your melbourne uni undergrad in 2 years if you overload after your first year and do: 4+1(winter)+4+3(summer overload)+5+2(study abroad/winter subjects)+5 = 24 and it's not even that difficult to pull off (3 in the summer is nowhere near as scary as it sounds) then you can go straight to law the next year and do that in 2.5.
but the thing is i think the focus on time to complete the degree is irrelevant in most cases anyway. most people don't finish as soon as possible because people enjoy university and like their time there. especially if you spend it doing things like joining clubs/going on exchange/volunteering/joining sports teams/travelling/etc. so if you really wanna overload you can do it in 4.5 but to be honest the argument of time is immaterial in the grand scheme of things. 1 extra year in a 40 odd year career will not mean too much and I think you would be better served enjoying your time at university and just picking the university and degree structure you want more.
I was considering law/commerce double degree as an undergrad but didnt like the fact you can only do subjects from those 2 areas in your degree. at melbourne you can do undergrad commerce with arts/science/anything breadth and then if i wanna do law later i can (but i've changed my mind which is another great thing about 3+3 model).
i really think you should just pick whichever uni you want to go to more/weigh the risks& benefits of going straight to CSP vs not locking yourself in