You are alone and witness a child collapse. you identify the child has no pulse and needs cpr. how long should you perform cpr before activating ems?

The answer would be: 2 minutes

Ideally, you have to activate the EMS as soon as possible. But it was not possible if there is only one person in the area. In this case, you should prioritize doing CPR as soon as possible for 2 minutes which is around 5 cycles. The chance that the child condition becomes improved will be much less if you delay the CPR.
However, you need to call for help even if the child condition not improved because you can't handle this condition alone.

Gravitational force i believe