Let's take this bug for example, of which the following fix exists upstream on the focal branch.
My current system is running bionic (Ubuntu 18.04.5 LTS) with kernel 5.4.0-47-generic (as installed by linux-image-generic-hwe-18.04)and I would like to get a rough idea of when this fix will be backported to my current system.
I'm going to wildly make the following uninformed guess... I figured there are 2 ways a kernel bug fix reaches my system.
If the fix is included in an ABI bump a patch, which means it will be available once kernel 5.4.0-x-generic(where x > 47) becomes available in my package manager via an update to linux-image-generic-hwe-18.04. Looking at the changelog for said package, it seems this happens roughly 2 weeks - 1 month (is this even correct??).
If the fix is not included in an ABI bump a patch, that means it will only be available on the next kernel release, i.e. 5.y.0-generic (where y > 4). This means that it will only be available when Ubuntu 18.04.6 comes out (tentatively February 2021).
So how do I know which pathway a bug fix will follow? Do my estimates make sense? What is the correct way of estimating how long I'll have to live with a bug provided a fix is available upstream?
Short version
I think this particular fix should be arriving tomorrow or the day after.
Long Version
So I can help answer the general case for Ubuntu packages but it gets a little more murky for the kernel and doubly so for backported HWE kernels. I've just gone through this process for two bugs I submitted, one for Open vSwitch and the other for GRUB.
If you find/fix a bug in an Ubuntu package, the default is to only fix it in the next major Ubuntu version (e.g. I found a bug in Focal's GRUB, it was only fixed in Groovy). If the problem truly is a bug vs asking for additional functionality, you can submit it for a Stable Release Update (SRU) which then will applied to currently supported Ubuntu versions. If the SRU is accepted, it goes into the "proposed" repository for 7 days during which time at least 1 person has to mark it as a successful fix and it has to pass automated testing. After 7 days, it then goes into the live "updates" repository.
Looking at your referenced fix, it references bug #1892417 which was successfully submitted for a SRU. If we look at the Focal proposed version we see that the code related to bug #1892417 is included. It also appears looking at the package's metadata that this build took place on 10 September so I believe that it should be hitting the updates repository in the next day (assuming no problems).
What I cannot tell you is, in general, the frequency of kernel releases as there is a lot of churn there. They may do releases every 14 days, once a month, or as needed. I also do not know when the updated Focal kernel gets backported to Bionic's HWE kernel. Comparing the release dates for 5.4.0-47.51, this last step looks instantaneous.