recommended limit on array sizes?

I'm using extended SQL queries (where I insert multiple columns with
one query) to limit the number of SQL queries that have to be made on
the server by storing every column that I'd like to insert into an
array $temp. Is there a limit on how big I should let the array get or
on how many columns I should insert at a time?

The largest number of things I'm going insert has around 2 million
entries totalling 40MB (it's in another DB).