Under Setup > Config, take a look at Pruning Rule almost all the way to the bottom.
From the help within alpine:
OPTION: Pruning Rule
By default, Alpine will ask at the beginning of each month whether or not
you want to rename your sent-mail folder to a name like
sent-mail-month-year. (See the feature Prune Uses YYYY-MM to change the
format of the folder to sent-mail-yyyy-mm.) It will also ask whether you
would like to delete old sent-mail folders. If you have defined Read Message
Folder or Pruned Folders Alpine will also ask about pruning those folders.
With this option you may provide an automatic answer to these questions. The
default value is to ask you what you'd like to do.
The six possible values for this option are:
ask about rename, ask about deleting
This is the default. Alpine will ask whether you want to rename the
folders and whether you want to delete each of the old folders.
ask about rename, don't delete
Alpine will ask whether you want to rename the folders, but won't ask
about or delete old folders.
always rename, ask about deleting
This means you want to always answer yes and have Alpine automatically
rename the folder if possible. You will also be asked about deleting
old folders.
always rename, don't delete
This means you want to always answer yes and have Alpine automatically
rename the folder if possible. There will be no deleting of old
folders.
don't rename, ask about deleting
This means you want to always answer no. Alpine will not rename the
folder. You will be asked about deleting old folders.
don't rename, don't delete
This means you want to always answer no. Alpine will not rename the
folder. There will be no deleting of old folders, either.