I am owning Lumia 610 and is there any way to know when My phone gets OS 8 update?
You cannot upgrade a Windows Phone 7.x device to Windows Phone 8. This can be confirmed by a post in the Nokia Forums.
Windows Phone 8 is a generation shift from Windows Phone 7.5 and will not be available for current devices due to the hardware requirements for Windows Phone 8. That said, an update will be made available which will, amongst other things, provide the new start screen to current Windows Phone devices.
The Lumia 610 was launched as a Windows Phone 7 device. Windows Phone 7 is based on the Windows CE architecture (CE stands for Compact Embedded- this was an operating system that Microsoft deployed to embedded systems such as mobile phones).
Unfortunately, shortly after Windows Phone 7 was launched, Microsoft went down the route of unifying all of it's platforms (desktop, phone, etc.) and hence the next version of Windows Phone i.e. Windows Phone 8 was not based on Windows CE but on Windows NT (just like the desktop Windows 8). This is why Windows Phone 7 devices cannot run Windows Phone 8 OS (different architectures).
To appease buyers of Windows Phone 7 devices, Microsoft released Windows Phone 7.8 as the final update to Windows Phone 7 devices.
If you haven't already upgraded to this version, take a look at this page by Nokia.