I need to get a British visa for my wife as we will be transiting through London on our upcoming trip in December. i need to get a land side transit visa so she can go out of the airport for our 9 hour layover.

my question is, how long does it usually take for visa processing in the US. we live in TX so i will need to send her passport in. i can't find any info online so i wanted to see if anyone can help.
our trip is coming up on the 23rd of Dec so i am trying to decide whether to just not risk sending the passport just in case it takes too long or go for it and wait out the layover.

thanks!