If you're using it as an adjective or adverb, the "a" has a short sound:
* The last time I saw her was yesterday.
* She last saw him at the store.
If you're using it as a verb, the "a" has a long sound:
* They last for hours. (to endure, to continue)
So, the word "last" can have both a short and long sound depending on its usage.