Ooh, topical question!
I suppose best way to answer this question is by asking one in return!
If you bought a fake for any reason except for price difference -- what do you expect to get from deal?
Would you buy a fake for any other reason than price?
Thinking honestly -- I can't think of any reason other than 'making a saving' for buying a fake bag.
After all, surely one of biggest problems with buying a fake bag is knowledge that it won't be made of same quality material or build? If it doesn't fall apart within first 4 -- 6 weeks, then you're lucky. I'd be awfully surprised to find your luck extending much beyond two to three months at most.
Even if bag doesn't literally fall to pieces, appearance of it will change hugely. Edges will fray and tear, stitches will pull and shred and general appearance of bag will dull, leather losing it's rigidity and ability to hold shape it's supposed to!
So bag that you bought for $150 as opposed to $600 has given you about three to four months use before it is starting to look embarassing, quite frankly!
Now, maybe you could buy three of same bag, save $150 and have something you wouldn't feel to ashamed to be seen with for up to a year. Just bin old ruined one, change your bag for a new one each quarter and you're away.
But that sort of puts you onto timescale where authentic designer handbag is just starting to be broken in.