Hello Anita,
Nobody knows an exact timeframe, Previously, it takes about a month to get LOA when status changes. However, i have read a couple complaints online with people complaining about not getting application in process status change within a month, this might be due to a change in MPNP system. Also, i have some applicants who had a status change and receive LOA with a month. Since your WP is expiring in December, trust me you will be fine, my guess is you will be getting your nomination between mid to end of august at the latest. Put your mind at rest, you have no cause for alarm.
Hope this helps.
Thanks,