The poll interval is variable. Here is an excerpt from the ntpd man page:
By default, ntpd runs in continuous mode where each of possibly several external servers is polled at
intervals determined by an intricate state machine. The state machine measures the incidental
roundtrip delay jitter and oscillator frequency wander and determines the best poll interval using a
heuristic algorithm. Ordinarily, and in most operating environments, the state machine will start
with 64s intervals and eventually increase in steps to 1024s. A small amount of random variation is
introduced in order to avoid bunching at the servers. In addition, should a server become unreachable
for some time, the poll interval is increased in steps to 1024s in order to reduce network overhead.