Welcome back

.
So first off, we're wondering what our window is for applying. I arrived in the UK on December 22nd of 2017, and my BRP has an issue date of December 2, 2017 with an expiry date of September 8, 2020. What is the earliest and latest date we can submit our application? And am I allowed to travel out of the UK while the application is being processed?
The earliest date you can apply is 28 days before you reach 30 months (2.5 years in the UK). The latest date is the expiry date of your BRP.
So, if you entered the UK on December 2nd 2017, you will reach 30 months on June 2nd 2020. 28 days before this is May 5th 2020... so your window to apply is May 5th to September 8th 2020.
In terms of applying, you just need to have submitted the online application between those dates. You do not have to have attended biometrics or have the visa processed by then. If your BRP expires before you get the FLR(M), your spousal visa will simply be extended under 3C leave while it is processing. Processing times are generally about 8 weeks.
You cannot travel while the visa is being processed, but you have a couple of options:
- apply in May or June before you go on holiday... if you don't think there will be enough time to get the visa using standard processing (8 weeks), you can pay £800 extra for a premium appointment which should give a decision within 24 hours and the BRP will be mailed within 7-10 working days.
or
- apply after you get back from your holiday in August... as long as the application is submitted before September 8th, you will be fine.