Hello,
The link You supplied is for C-series (SRC) product, not M/T.
It is also spelled out in URL itself.
For RMON alarms supported on M/T-series (and MX), please visit this link
http://www.juniper.net/techpubs/software/junos/junos94/swconfig-net-mgmt/alarm-entry-attributes-configuring-junos-nm.html#id-11462434
One thing to note is that You won't be able to define "stay there" interval explicitly although You can play with sampling interval
http://www.juniper.net/techpubs/software/junos/junos94/swconfig-net-mgmt/alarm-entry-attributes-configuring-junos-nm.html#id-11496404
Other possibility is to augment the RMON with SLAX script to collect consecutive measurements over Your desired "stay there" interval and then, if "stay there" interval is exceeded, send a trap/syslog message.
HTH
Thanks
Alex