Re: How long before.....
The Moon isn't exempt - it falls under COSPAR (COmmittee on SPAce Research) planetary protection guidelines which are linked to the 1967 Outer Space Treaty. COSPAR guidelines go from zero (essentially missions to the Sun) which require no special biological contamination protections through to Category V which is reserved for sample return missions to bodies where there is a serious risk of contamination by Earth microorganisms.
Since the Moon is blasted by UV, the Solar Wind, has a massive temperature range and no atmosphere, it is highly unlikely that any organisms or complex organic molecules will be found there. Under COSPAR, missions to the Moon are considered Category II - destinations which might contain information about the origin of life but where terrestrial contamination is unlikely to interfere with data.
As for the COSPAR II requirements - basically fill out a form saying what you plan to do and any potential risk of contamination; then another form if you should do anything bad when you were there. The probes themselves don't need any particular sterilisation before launch.
TLDR - crap on the Moon if you want, but fill out the paperwork afterwards.