If you are not in MSA currently (pure c4d user of last version without MSA), you can buy MSA anytime before R20 official release. Current state of MSA isn´ t lock to time, but count of releases, so MSA means 1 new version for lower price then "normal" upgrade, 2 installations (not running concurrently) and Cineversity year subcription.
But still is locked to time... :), because when you purchase today, it´s year subscription and takes for 365 days. ( So, ending the same day, but next year and still are there 2 mounths to release new version). In this "hole" you can again decide to buy MSA (you are still allowed) and continue in upgrades for lower price...If R20 release and you ´ll be without MSA, first you need buy "normal" upgrade to R20 for higher price (like MSA) and then you´ll be allowed to buy MSA for R21 for instance...