If you're on a losing streak chances are you're demoralised and not thinking right. Go and do just about anything outside of the game. I like to watch videos online, drama, animations, etc.
Usally they say take a break because loseing could make you angry, demoralised e.c.t and this could lead to you making mistakes. You can watch a video, read a book or even play in a different room like duel.
I usally take a break after 6/7 losses but its different for each player.
I take a break when im tired of facing the same decks over and over and over
give it a day or two and there is a chance the popular deck changes and you get some variety
I take a break after back to back losses, or a loss that ends a streak. I play a different game for a bit, then come back. Usually does very well.
Either 2 non 50-50 losses in a row or 3 out of 4 losses or smth
Everytime you lose, close the computer, go to a 10 mile run while thinking over why you sucked, and try again. Not good for your playing but at least you'll get in shape.
If you're constantly losing match after match, considering refurbishing your deck to the current meta that consistently keeps raping your deck