Every response from the API can be cached for up to a minute, so making the same request multiple times in that time-frame typically won't return different results.
We make note of this in the throttle documentation:
While not strictly a throttle, the Stack Exchange employs heavy
caching and as such no application should make semantically identical
You can get data close-enough to "real time" for most practical purposes. See the StackHose app, for example.
Read the page about the API's Throttles and Quotas. From that page we can deduce:
The maximum, burst, request rate is 30 requests per second, and this risks getting your app shut down.
Your app will never be allowed more than 10,000 requests per ...