Not long, maybe a minute including POST and all the pre-boot stuff. But sitting down and having everything exactly the way I left it gets me back into "flow" much quicker than having to start up all my applications and get them back to where I left off. Yes in theory hibernate/sleep does this and it usually does but sometimes there are weird quirks, in particular ssh sessions, remote filesystem mounts, and other types of stateful connections are usually borked, and I'd rather not worry about any of it.