I want to know the uptime since the last wake from standby.
uptime only shows the difference between current time minus the last startup time.
In /var/log/pm-suspend.log, look for the last line looking like this one:
That's your last wakeup time. You can calculate your uptime since then the way Paul suggested.
What are you using to initiate the standby?
If you can use a script, then after the line
you could have the line
Or something similar. This would mean that the first thing the machine did when it woke up was to write the current time and date to a log file (n seconds since epoch).
Search for the last occurence of the string "PM: restore of devices complete" in /var/log/messages. If your machine has been up too long, then the log may be rotated, though.