When's the best time to change your job/career field

OT, when is a good time to change your job/career? I assume most of you who have your current job normally stick it out OR when you find a better job and/or opportunity comes will you jump ship for that position?

A little background. Single, no kids, engineer for a contracting company that's seen better days. Long while ago I was unemployed coming out of college and got brought on here. In the process of getting on my feet, was fortunate to have gained 5 years of experience however the industry (nuclear) is not what I prefer to work in. Too much red tape, can get boring, and stagnate.

For those who know about the engineering job field, some companies do not like hiring someone who's spent x (single digit) years in one field because they feel that person is used to that field and the information in it. Would rather train someone fresh. But for other engineering firms, they do look for engineers with 5+ years of experience (more so in a related field). Either way, when is the good year/time mark to get back on the search trail for another job experience wise and time wise? Should it be earlier? i.e. before gaining that experience? And if changing to a different area in my field (Power to tech or nuclear to power) is that unattainable or more common?