Used to play it around 90% in TV mode and only occasionally in portable when I was travelling or desperately wanted to keep playing when I went to bed. Over the last year or so though it's swung massively the other way - I love playing it in portable mode now as it's just easier and more relaxing I find. I answered 50/50 above as that's a fairer reflection of my overall time since I bought the Switch.
The real answer is it depends what game though - major AAA titles I will play docked because they will look and sound better whereas Indie/Retro/handheld-centric games (such as Animal Crossing, Links Awakening etc.) I play in handheld. This is why I love the Switch as a product. I treat it as both my main TV console and handheld one - it gives me the best of both worlds.