Enabling ART on android How much time it takes to enable android run time on Nexus 4.4.2 kitkat. 
I have near 90 apps installed.

I start it and screen comes that displays optimizing 1 of 90 apps.
After it got completed (all 90), screen with 4 colored circles revolving appears(i think that's the boot screen or something same when the phone is switched on), but keeps on for more than 1 hour.
Sounds like you are in a bootloop.  If you've enabled android debugging, you can probably use logcat via adb logcat on your computer to read what's happening during the boot and figure out where/why it's crashing.  You'll usually see a crash message, and if you're in a boot loop, you'll frequently see a repeated cycle of text.

If googling samples of the log doesn't help come up with an explanation/solution, you might copy/paste that text, and (after verifying that it has no private information you don't want shared), paste it to a site such as pastebin.com or gist.github.com so that it can be easily shared.

Next, visit IRC (you can do this from the web and try one of the android support channels such as #android to see if you can find someone to help "live".  You can show them the link to your bootloop log, and they will hopefully have a suggestion for you or redirect you to more information.

You can also update this question with a link to the logcat and maybe someone can help you as well.

As a last resort, you can always boot into recovery mode and do a factory wipe (or merely wipe the cache), which may help as well.