By using scripts to change existing queues. Each client gets a queue that shapes traffic based on source/destination IP address. Via the scheduler a script fires that changes the bandwidth limits of the queue at certain times of day.
This sqripts found date from billing and edit a current simple queues for all needed pptp session
for example:
[admin@MikroTik] /queue simple> set 1 limit-at=512k/512k max-limit=512k/512k
and when day is begining set old state
[admin@MikroTik] /queue simple> set 1 limit-at=256k/256k max-limit=256k/256k