The phrase "a major waste of time" is correct and usable in written English.
It can be used to express that something is considered to be very unproductive or not worth the time spent on it.
Example: "After attending the meeting, I realized it was a major waste of time, as we didn't accomplish anything meaningful."
Alternatives: "a complete waste of time" or "a significant waste of time".
The main reasons for their social media sabbaticals were not having enough time to dedicate to pruning their profiles, an overall decrease in their interest in the site, and the general sentiment that Facebook was a major waste of time.
"If I had gone through all those years of training and didn't use it, I would have thought it was a major waste of time, money and effort," says Karen Allendoerfer, a Princeton graduate who earned a Ph.D. in neurobiology from Stanford University and who is now the mother of a toddler.
These sites can be a major waste of time.
When nailing in the rails, measuring each post can be a major waste of time.
Clearly, this leads to a major waste in resources for the majority of the time.
There were those who said they believed that not only this conference, but all such attempts at global statements on major issues, were a waste of time.
"Turns out that soliciting an autograph from every single major league team is a colossal waste of time after all!" At least he got an article out of it 17 years later.
Merely hefting the basket up on the belt with the contents still inside is not only a major pet peeve of cashiers, it is a waste of time for you.
Blogging is often seen as a waste of time at major research universities, where those seeking tenured positions "progress in their careers based on their research output.
It is humbling to recall that when the idea of deciphering the human genome was first proposed in the 1980's, most of the major figures in biology deemed it a waste of time, talent and resources that would generate reams of useless data.
