Arugula and spinach are of similar culture. You can plant then when it has nice sized leaves cut it all, or you can pick individual leaves and let the plant grow taking leaves as you need them. In either case there will be a point where it will no longer make good leaves for you.
I like to plant early in the Spring and then plant every three weeks up till late June when it gets hot. These crops do not do well in our hot summers. Then one can plant again about Sept first. You are in a different zone, likely you don't get the cold winters I do here. Have fun!
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-