not sure how to configure an alert for that ....

but you could set up a job to poll this memory counter say every 1 minute and put it into a place holder table with a timestamp. then you could check last 2 values to see if it exceeded your threshold ... if it did send an email - database mail ... telling you that threshold exceede in the last 1 minute.

last step of this job could be to delete data older than a day or so from your place holder table to keep it under control.