The value is in theory precise down to the millisecond, but I am skeptical.
Does that number tell when the packet was transmitted, or when the connection began or ended?
Note: Subsequently I noticed that there are flow-start time & flow-end time fields.
Best Answer
There are both in Netflow v9: according to rfc3954:
So you can have both a sysuptime for a flow or the whole device. Looks like your original question is about the header of a Netflow packet, so that would be the uptime of the device.
One last thing: the sysUptime value can only record about 49 days, since it is a 32bit value.