It depends what you mean by planning a trip.
For me, in the last four European trips and in the forthcoming trip, planning the places we were going to visit and how to fly or drive there, have taken somewhere between 20 minutes and half an hour - on two occasions during chats with chaps at Qantas when I was using FF points. That's the easy phase of planning the trip.
The hard part of planning for me is the hours then spent picking the hotels on Booking.com (and sometimes Hotelclub). How much does it cost? How far from the centre of town? How big are the rooms? What sort of reviews have Australians given it? Does it have car-parking and how much does it cost, when I'm driving there? What do people think of the breakfast?
Then having narrowed the hotels down to several feasible solutions, I bring my wife into the decision making process.
I spent many hours back in March picking seven hotels on Booking.com (after accepting three hotel offers from my travel agent). The worst town was Munich - no parking in the main part of town, but there was plenty of parking in the hundred hotels to the east of town. Only problem was that it turned out to be a sex district, with all the reviewers complaining about the noise and seedy looking types wandering around. and then stacks of otherwise good-looking hotels turned out to have rooms of only 14 or 16 square meters - way too small. So I booked a hotel out on the western side of town, with free parking, but requiring a train ride into town.
Unfortunately, one has to take Tripadvisor reviews with a pinch of salt, as it is not unknown for lousy hotels to pop some very positive reviews in there for themselves - something that can only happen on Booking.com if the reviewer actually books a room and pays for it.
Regards,
Renato