Some pools offer fixed diff ports but vardiff is most common. In my experience with CPU mining being able to set the starting diff is very convenient.
Many pools won't adjust until you actually submit at least one share so if the starting diff is way too high for your miner you're stuck.
The typical target for most pools is around 5 shares per minute. Vardiff pools also have a minimum difficulty, a weak miner may hit that limit and submit
fewer shares than the target. Five minutes without a share is likely to result in a stratum timeout and means you should probably find something else to
mine or mine with.
You can find the math here:
https://en.bitcoin.it/wiki/Difficulty