I'm honestly curious given that I suspect the Wii U won't provide the best example of sales frequency: how often do companies have a sale on a PS4 or Xbox1. 

Does a company do so every month? Every few months? Is it whenever they feel like it?

This is from a Switch owner who enjoyed the surprise fun of Rayman Legends from a Ubisoft sale and am curious about the potential of that for switch third party titles down the line. 

That title is definitely missing some words.

How often do sales happen on psn/X1 though? there is constantly at least one sale running on psn, currently there are 3 ongoing in the eu market. digital flash game of indies, resident evil franchise sale and a may savings sale.

Would love to know how Nintendo are going to be moving forwards with sales.

PlayStation Store always has AAA titles on sale. It is a never ending cycle of games going on sale for different reasons. Game of the Year sales, Flash Sales, Publisher Sales, Genere Sales, Best on PlayStation Sales, and on, and on, and on. I have picked up many games for less $5 over the last 3 1/2 years.

It has been really nice, because there are games I have to get day one, games I will pay $30 for, and games that if it goes low enough I will give it a try. So far this gen I have over 300 PS4 games, and I have over 200 Vita games. Like I said, stuff is always on sale, and as long as you check the store each tuesday, you are bound to eventually find what you are looking for at a steal of a deal.

The X1/360 has a few titles on sale every week and slightly bigger sales about once a month and even bigger sales about once every three month.

PSN basically every 2 weeks change the sales except for one game that is a weekly deal and changes every week.

Xbox one has weekly deals.

Sony has more on sale but also take longer to change while Xbox one sales change every week but less is on sale...

Nintendo don't know really.

I honestly think PSN has too many sales. I wonder about how much profit that developers, especially smaller ones, are receiving or the level of reinbursement that they receive from SONY if any.

Dunno for consoles but on Steam quite frequently. Not crazy stuff like 80% but 10-20% frequently and 50% in regular big sales.

All the time on PSN. There's actually a new sale every week. Current sales on PSN EU/AU:

May Savings
Resident Evil sales
Deal of the Week
Save up to 65% on PS3 and Vita games