I've heard it was upwards of ten years between the two in Trek's timeline. I always wondered this, mainly due to the fact that in TMP, the Enterprise, now refit, is shiny and brand-new, but by the time we get to The Wrath of Khan
, and even later, The Search for Spock
, the Enterprise is said to be old and with the Admiral even saying, "We feel her day is over."
I believe the old book, the Star Trek chronology is the one that mentioned the ten year gap. Was it really that long?