CH inhibitor life and sealed vs. unsealed systems

Hi,

Please can anyone tell me how often the CH should be drained down and
the inhibitor replaced? I've got two years in my head but I might be
thinking of car antifreeze (the blue rather than red version)!

I've been using fernox but their web site is a bit vague on the issue
and says to buy a tester. Inhibitor is readily available but I don't
see anyone selling tests and I wonder if I did, whether the test would
cost more than the inhibitor!

I can't remember whether I used F1 or MB1 last time, if that make a
difference, I have used both in the past; buying whichever was the
best price in the past at the time.

It is a vented system. Would the interval be longer in a sealed
system? I imagine the F&E tank constantly allows oxygen into the
system whereas a sealed system contains only a finite amount?

Is it worth me converting the system to a sealed type? What is
involved in this? I know many here like sealed systems and say that
there is only a finite volume of water so leaks are limited, but what
other advantages are there? Some would say no need for a loft tank but
to be fair, that hardly takes up any room.

Regarding the drain down, is it worth using a cleaner beforehand like
Fernox F3? I know I used one when we first moved in and didn't know
the condition of the system but since then we have changed a lot of
the radiators as we have redecorated the rooms, so I am quite
confident that the system is in good shape now. Is cleaner only needed
in bad / dirty systems? Fernox say to use it but they have an interest
in generating sales!

TIA