How long before your campaign is not profitable

It really depends on the campaign / angle / size of demographic. The more detailed the targeting, the higher the ROI (usually), but it won't last as long (Since you have a smaller demo).

I try to shoot for a demographic above 2 million (Paid facebook advertising). With a potential reach of 2 million, I can sustain multiple accounts / duplicate campaigns for a month or more. I can expect to spend at least $50,000 before the roi drops too significantly. As the campaign spends more, it starts reaching the same people, which causes cpc to rise, ctr to drop, and roi to fall.

Now with an extremely targeted angle, my demo might only be 750,000. The roi might start off at 500%, but it will drop very quickly as I scale. I might only be able to spend $15,000, and average 150% roi. It usually makes more sense to have a higher demographic / less targeted angle, lower roi, but be able to scale and spend more.

I use facebook paid advertising and after 48h I have sales,sometimes it takes 1 week but in about 2 months I make $200/ day and it goes as long as I want,if I have a good niche.
Choose the right niche and half the battle is already won.
I literally spent years chipping away at a niche that was close to my heart, but it wasnt
ever going to give me the results I deserved.So forget things you love,this is not hobby,you
have to make profit as fast as possible.
The only way you can fail in this business is to chose wrong niche.Why?
Beacuse you can't afford to lose money.

Niche is good if you have all this....
1.Go to Google keyword planer.Click search for new keyword phrase.
Type main word,you will get how many people are searching .
Main word will be broad,you need to narrow it down,so look
avarage monthley searches.It should be Low competition and between 20,000-50,000.
Pick all of them in that range,collect word phrases and go back to Google.
Do you see advert? If yes,it is a good sign that money is in that niche.
Under search bar is About and number that tells you how much competition
you have.Up to 2,000,000 is nothing really.
Go to amazon to see what is being sold.
If you find books and magazines you are ok.
Go to clickbank to find products.If you see 3-4 you are find.
Just pick one around $30.
Social media-join groups on Facebook,what they like or not and so on....or
to save time click on FB arrow pointing down,top right corner on you FB page/profile,
go to Create ad,click on ads manager-audience insights.On the left side 'Interests'
your keyword phrase to get demografics.Click Activity and first on the right is ads clicked to give you idea if people are clicking on those ads.Page Likes tells you what else they like .
That helps you to offer them another product in the future.
Now you have your niche! Test your ads to get better results faster.
It works for me ,it will work for you.
Don't give up