Instead of implementing a delay that counts turns through a loop, configure and use a timer.

Then write a loop that continuously checks if the timer is "ringing" or if you have received any characters on the serial port. If you have received characters - parse them and decide if they represents a command to break your delay. If not, wait until the timer has ticked long enough - or overflowed enough number of times - that your delay should time out.

You may also process the serial data in an interrupt, and let the interrupt set an abort flag if it receives a specific character.