As Andy says: "There is no hard rule on this." It totally depends on cash reserves, empire size, future growth (which will probably increase your ability to generate more profit) and so forth.
Some additional context on how you could approach this question:
For newer players with less cash reserves, a gradual approach is probably best. Look at the requirements to build interceptor or defense helicopter wings, buy the gear to build several (with excess ammo) and watch for when your gear gets purchased and delivered. You can see what's required for each unit by attempting to set up these wings without the equipment. It will show you the default builds for these units.
Step by step to see requirements: Click: Army > Military Units > Air Wings. Then click setup new air wing. Select the wing whose requirements you wish to see and click next.
After weapons are delivered (which can take so