From what I understand a lot of this comes from the type of insulation you're using and the way you store it. If you're using continuous filament insulation like Apex then it has far more longevity than the Primaloft style, which will pack smaller but also degrades more rapidly - to the tune of losing about half it's loft after a few years standard use. If you have any Primaloft gear, it'll last far longer if you hang it in a wardrobe when you're back home and try to wear it instead of pack it up.
All that said, for inclement climates synthetic insulation is now regularly at the 600-700 fill power range equivalent to down (when it used to be around 500) and for the UK and Scandinavian climate gives a robust product which isn't going to lose warmth due to the moisture, particularly if you're wearing it all day as an active piece of gear.