AGB, I believe it is once a game day. I have been trying a little with no success yet, but a nice side benefit is that using Crabato Juice and playing musicblocks for crabs is a NICE way to earn currants fast!
I can't be certain but it seems to me that many more people are now making Crabs Happy. I and some others I know of have been working on this since early in the first Beta with no luck until the last couple of weeks when we all were able to make a Crab Happy. Is it coincidence or have the Devs tweaked this Achievement?
Of course, it might also be due to the holidays and the fact that fewer people were playing making this easier to do.
It's because, since the discovery that SB-1 is included in the rotation, those with the block (our OP among them) have been playing it regularly, which prevents it from perpetually being the "least recently played" and allows others to get the achievement. Oakland was able to make a crab happy yesterday with SB-1, which confirms that it has not been removed from rotation.