Okay, i found it. In case you want to see your masternode's uptime via command-line desire cli, you can run
# all masternodes
desire-cli masternodelist full
# specific masternode
desire-cli masternodelist full | grep ''
# example result
"41db00d59d42688aaf5fa6d6249ce99778c899467ccf4c98dddf99451aadd6f3-1": " ENABLED 70208 D98oPyZTVesLE7BHB4wWAt3onTKk7gH2Mu 1513860932 2368705 1513814687 36278 IP.ADD.RE.SS:9919"
I suppose that the value next to your address is a last seen timestamp, and the next one is uptime.