The Actuarial Studies/Law double degree will take you 6 years, assuming you want to gain all exemptions for Part I and Part II accreditation.
I know several people undertaking it. the only real downside of doing it is that you do Law subjects in first and second semester of 1st year, and so you are taking a subject different to most other people doing Actuarial studies - which isnt an issue unless you wish to transfer to another double degree.
The beauty of the current way Actuarial studies works is that other than the law double degree (And the accounting/computing double degree,w hich i think they are scrapping) all other double degrees have the same subjects for first year, so transfering between them (or from single to double degree) is as simple as filling in a form at the end of the year, and you are at no disadvantage.
Check out
Recommended Programs of Study - Department of Actuarial Studies - Macquarie University to compare teh programs of study as they currently are for different double degrees.
However notes, as PwarYuex said, that there are changes to this for next year, for example ACST151 will be replaced by 152, ACST211 will be replaced by 212, and other changes might take place, so this should only be an indication of what youa re going to study, and not final.
In terms of workload - actuarial studies is quite a bit, you will be going 5 days a week for first year at least (not full days i should stress, facetime about 16 hours for 1st semester, 20 hours 2nd semester)./ However if your willing to study like at school (As in do a bit every night) then you should be fine.