auto_increment will grown until it reaches the limit of the value that that column can take.
For an unsigned int, the limit is 4294967295 that's 4.2 billion rows. By the time you reach that number, you'll have dropped MySQL and started a using a real rdbms.
It may sound odd to use higher numbers with holes in the sequence instead of lower sequencial ones, but those numbers are for internal use only, so it makes absolutely no sense to re-number your items in any way.
