Higher doses give greater effects, but also greater side effects - that is, you're more likely to enter the Dark Night with everything that entails, and if you're not ready for that, it can be a problem.

Personally I find longer sessions are always better - aside from making more insight progress, they just seem to clear my mind out more and make it less sticky and calmer.

If you can concentrate for more than a minute once or twice every hour, you're doing amazingly well. That shouldn't limit the length of your meditations - it's just a reflection of how clear or cloudy your mind is. Think of your mind as the weather - sometimes it's clear, sometimes it's cloudy, and it's not in your control.

If you're aware that you're only aware for a minute or two, that's good in itself.

Not sure what you're doing, but I would recommend some kind of structured style to keep you aware.  Breath counting, noting, noting out loud, body scanning, whatever.

There isn't a "right" amount of time for meditation.  An hour is great in a sense, it's more than most people put into it, just get the quality up.

It might be that you try to "concentrate". If the mind rebels you cannot force it do things. Try to remain mindful, but relax the focus and allow some movement of attention and wider picture. Try not to constrict the mind where you want(ed) it to stay, and relax. Practise first continuity of mindfulness and then the mind will be easier to settle somewhere more locally