can i use the strimmer for long periods to start with or just for a short while
Use it like you stole it. It needs no break in, go for it! If it were mine I would add a little more two cycle oil to get mix to about 40 to 1 on the new engine. It will smoke a little more, but gives better protection for the piston/rings.
How long will i use it at 40/1? what else should i look out for?