All

I travel 2 x 4 sector round trips each month mainly EDI-DOH-DXB but others too.

I am offered these upgrades almost every time but it IS dependant on J space. They will not offer extra miles/points for it as it's a fraction of the cost of buying J up front! Certainly wouldn't equate to extra miles on another FF programme (e.g. BAEC). If you do buy the upgrade, then dont be confused when the send you the re-issued ticket with 'I' booking class. You will still receive the original booking class miles and points.

Timing:

I am sent these between 14-5 days before my journey, sometimes even offering return sector already - i would assume that's dependant again on whether they are likely to fill them with full fares or not.

Prices:

These are prices that i've either been offered or paid - clearly LHR is far more expensive as always.

DXB-DOH / or DOH-DXB - don't ever do this! - £397 one way (as it's marketed F)
DOH-EDI - £350ish
EDI-DOH - £430ish
DOH-OSL - £339
DOH-LHR - £554

As you can see it also has to take into account the extra airport tax for being J (i.e. even if i upgrade in miles from UK i pay £70ish tax - higher than DOH for example)

You are paying for service, comfort, lounges (proper business Al M if you've upgraded your sector from DOH), extra baggage and all the trimmings - just NOT the extra miles or points. Given the prices they quote you it would just make no sense!

My advice - if you can afford it - pay it. It's well worth it for the above reasons alone.