Thus 1-2 years is a good idea and thus never stay 7 years in your first full time job!
I agree with that. All my jobs have been 2-3 years on average. Least was 1 year, most was 4. That's not necessarily moving employers, but certainly moving between departments or getting internal promotions within the same dept.
Personally, I think 2 years doing the same thing is long enough. It's long enough to learn the skills etc to do "that" job, and unless you have absolutely no ambition, there's no point in just doing the same thing over and over again, month after month, year after year.
Most training/apprentice jobs will have some kind of progression built in, i.e. may be the same place/same "job" as such but doing more technical/complex work after you've mastered the basics. That's fine whilst you're learning and progressing, but when it starts to get stale, it's time to move on.
If you're in an organisation which lets you progress, i.e. different depts or move up into supervisory/management roles, etc., then fair enough, no need to move employer (but still a good idea to move anyway especially if pay and other benefits aren't increasing commensurately), but if not, then best to move to other employers to climb the career ladder.
I did a mix of both. First job was entry level trainee, where I developed quickly and within 18 months or so was at the same working "standard" as co-workers, but pay rises were slow and I was earning a lot less then older work-mates, just because I was young! I kicked them into touch and worked for another firm. First year was in a pretty awful working condition (branch where the manager was awful), so I asked for a move and was moved sideways to their head office, I did 2 years in one dept, then moved to a different dept, so 4 years there in total, but 3 different roles. When I qualified, they didn't increase my salary to a "qualified" level, so I moved to a different firm and doubled my wage. After a few years there, I moved into a different industry but doing the same job. A couple of years there, and I moved back into first industry, but at a senior manager level. I've never regretted any of the moves - at each job I'd put my all in, but if they didn't offer progression and package accordingly, then I moved.