If the system is working properly after starting the needle should indicate a reasonable charge rate ( 3/4 or so to the right) at a higher RPM ~2k + or so as the alternator is replenishing the battery. If the motor has been cranked for awhile when starting the initial charge rate may be higher.
After a couple of minutes the needle should be straight up or very close to it indicating little or no charge.
Be aware that at very low engine speed with higher draw accssories the needle may show a discharge ( left of center) until the RPM increases. It would then show a charge (Right of center) until the battery is once again replenished.

A constant charge rate is indicative of an issue somewhere in the system.