Well that question could have a few different answers but I’ll try to explain the traditional definitions. Back in the day when the album was king, you could get about 46 minutes on an album, maybe 23 minutes per side “max” because of the physical limitations of vinyl. With the advent of the CD format, you could get a lot more material on an “album” although most record companies wouldn’t allow this because it would have cut into profits significantly. A music album today? I guess it’s whatever collection of songs someone chooses to call an album.
An EP these days it seems can contain as little as 3 songs but would usually have at least five. The term doesn’t really make any sense in a streaming, download driven marketplace other than for marketing purposes.
A demo is a very broad term and hard to quantify anymore because there are so many demos being released as “MASTERS”. It use to have to do with unions and signatory agreements with labels but there are so many non union music recordings released these days that that definition of a “demo” seems to be fading away, in my opinion.
There are also demos for artists, producers, labels and any other entity one may try to get to publish their music. Generally speaking, these would be music recordings done much cheaper and quicker than master recordings for mass consumption; although, I’ve heard many publishing demos that sound like “full blown” records. Knowing who you’re pitching demos to has a lot to do with how far into the production process these recordings should be taken.
I guess a book could be written on it but I’d rather write a song.