I have done three times in a calendar year, April, September and November twice, depends on who you are standing in front of at immigration, as a by the by I have heard that you should avoid the women IO's as they can be more officious, but I have no evidence.
Good question, I always make sure that I have my latest prescription with me, but I've never been asked for anything (100% know what will happen the next time I go 😂)