SubQ is not the ideal but if you must, do small amounts and very slowly. The absorption rate is NOT the same as if you injected into muscle and it takes longer to absorb. Hence the reason why testosterone works well for women through subQ. I have never done primo SubQ and wouldn't do it due to many factors such as pain, lumps as it is not water soluble like GH or Insulin, etc. I do all my pins IM. Feed the muscle directly type of a thing.
As for dosing and duration, given you are already on test, start your primo dose at around 25-35mg a week to start. Work up to 50mg a week max split into two shots. That is a typical dose for women, and no need to go high. Less is more, use the smallest dose to avoid undesirable sides. You can run Primo for longer periods as it it a long ester and takes a while to kick in. I don't recommend running primo for less than 12 weeks. My recommended ideal is between 12-16 weeks. In some cases even up to 20 weeks.
Primo Enanthate has a 10-14 day half life so you could technically get away with injections once per week but the ideal is twice a week. If you have Primo Acetate, then every day is recommended and usually that is in oral form, rarely you see it in injectible form.
Primo is a "feel good" compound and you get quality lean gains from it, hence the reason why you can run it for an extended period of time. Typically around week 8, you start feeling it and seeing it do it's magic. I wouldn't waste primo for less than 12 weeks cycle.
If you want faster acting and quicker runs, there are other options available. Primo will be one of the only ones that will give you true lean muscle gains that you will keep, hence the reason why it takes so long for it to build up and show itself. I love primo for that reason.