You could create a script that monitors the number of bytes, and/or packets that is transmitted in either direction, and then change the queue's speed once the 10MB has passed through the queue. The only problem with this approach is the overhead associated with monitoring the traffic on the router.
An alternative (off the Mikrotik website here -> http://www.mikrotik.com/mfm2.php?category=3
The Dynamic Bandwidth Shaper is a cloud based network management service which tracks subscriber usage and dynamically adjusts queues on devices running the Mikrotik Router OS. Designed specifically for WISPs, rural ISPs, and the hospitality industry to take control of subscriber IP based entertainment service demands by instituting a tier based service model.