How often do you NEED to withdraw?
That is up to you.
But Upwork is not a bank.
Your money CAN disappear from your Upwork account.
If money is left in an Upwork account too long (six months), then Upwork is required to get rid of it.
As a practical matter, you live in California, and you can withdraw money at any time, in any amount, for free, to a U.S. bank account.
So why would you leave money in your account?
There is nothing to be gained from doing so.
You do not need to make regular withdrawls, there is a few options such as automatic withdrawl which allows you to setup a payment threshold before payments are made.
However, i would suggest you make sure you have a valid withdrawl method on file as after money has been in your account for 180 days Upwork will automatically withdraw it if you have not already done so!
If the 180 day period comes around and you have no withdrawl method on file i am unsure what happens, but i would not like to test it! I'd much rather have my money safe in my own account