When you turn your 5mg pen from 0 to 1 in the window it is 60 clicks to deliver a 5mg dose.
If you only turn 30 clicks you will get a half dose of 2.5mg from your 5mg pen.
When you do it the first time mark the 30th click with a biro in the window so you can return to it easily each time.
You will be able to easily get 5 doses out of the pen within the 3 day in-use period - on days 0,7,14,21,28.
You will be able to get up to 8 doses before the pen locks on days - 35,42,49.
Personally I keep in fridge and have done up to 6 doses/35 days, just 5 days out of in-use period, as I see that as very low risk. I feel less comfortable with longer as risks are unknown.
Also be aware some providers will question you not ordering regularly. Mine lets me away with up to 6 weeks between orders but they do usually ask why and I tell them I missed a week due to holiday, illness, forgot, travel etc. Because it is a lower dose they don't tend to question gaps too much.
Make sure you have enough pen needles. Buy the same brand/length/gauge as your provider sends you (details usually on the removable tab of the needle). You can buy boxes of 100 of these fairly cheaply from anywhere including Amazon.