It would not be that long. As it is now blocks, on average, are much less then the 1 MB size limit, and pools will often broadcast blocks that are well under this limit to avoid risking their block getting orphaned. However this risk would be decreased with the new, temporarily longer block time plus the additional TX fees included in many transactions would result in the blocks generally being much closer to 1 MB, if they are not all filled to 1 MB exactly.