platform is irrelevant. game type is what matters here.
I am working on a concept for now of a PvP game which would be limited to phones and possibly tablets.
this really is largely irrelevant. whether you play skyrim on a PC or a tablet, its should last a long time. whether you play galaga on a PC or phone, each "round" is typically a few minutes at most.
i'm modeling it after two styles - tower defense and card games.
this is whats important - game type. a phone is just another platform - albeit somewhat less powerful than an alienware rig.
how long it should take is really up to you. who says tower defense should only take five minutes? that's not much of a game if you ask me.
how " casual" vs "hardcore" your game is will probably be the deciding factor. for casual, you want something they can pick up quickly, play quickly, and put down at any time. but as long as they're playing, you want them to be able to continue playing. so a design like galaga with 100 levels is better than something with just 5 levels. insufficient content / gameplay hours leaves a bad taste in the payers mouth, "i got ripped off! its too short!" for something more hardcore, you'll want longer term gameplay most likely, with the ability to save and resume. but since its two player only, playing a longer game - possibly over multiple sessions - doesn't really fit well. i mean, when's the next time you and your buddy will both be able to continue the game?