This is entirely up to you! If you're just starting out, we would recommend starting out with something between $250 and $1000, depending on your budget and your overall campaign goal.
If your campaign is in response to something recently in the news or is receiving a lot of media attention, we highly recommend investing a few thousand dollars or more as soon as possible. If you wait until the news cycle has shifted to something else, the ads will not perform as well.
If your budget for marketing is limited, seek out a donor who is willing to help you invest in ads. After all, who wouldn't want to see their donation multiply into more donations?
Remember: more $$ invested = more $$ donated (as a general rule of course!)