I have not read Fifty Shades of Grey, or any similar type of novels. The subject does not appeal to me, but good luck to the authors. I am in favour of any author who encourages people to read.
I was wondering how large an appetite readers do have for erotica, and how long the trend will last.
I might be wrong but I believe that erotica will be short-lived in comparison to other trends.
You can write a juicy novel about sex. The susequent novel has to be juicier. The third novel has to be more juicier to maintain the readers interest.
By then I believe the plot and storylines will have reached orgasm.
What else will there be to write about?
My initial thought when I saw this question was "I suppose that it will last for as long as people are 1) interested in sex and 2) reading about it." (IE: Pretty much forever).
However, my second thoughts are that it will probably not last in the mainstream press and that FSOG will eventually be swept aside by something else once the sensation has died down.
Having written a few unpublished novels which feature sex scenes and having toned down several in my present novel which I aim to publish if all goes well, I have always found that sex scenes are a big challenge to write well.
Generally, I use this rule of thumb to guide me: They are alright if they support the wider storyline and they slot into it naturally. If they are the only or main storyline, or if they are forced in there for the sake of it, that is when I personally find they start to get boring/annoying/distracting.
I'm not sure that it will last on the mainstream shelves per say but I must admit there is an underground community of erotica fans on the internet - you just have to know where to look!
Many Fan-Fiction authors do dabble in the erotica side of existing characters from other fandom, some sites are dedicated to Adult audiences such as AdultFanFiction.net that is solely based on MA content, be it erotica or excessive violence.
fictionpress - I believe - have an erotica section but I'm not sure if it's changed because of tighter age and content control (as it's the partner site of fanfiction.net)
I must admit, I've read FSOG and I find the erotica predictable with purple prose - as standard - and to put it bluntly, 'coming at the same time as your partner or on command is just fanciful and CANNOT happen EVERY F-ING TIME!' (Sorry for my out burst but I just needed to get that off my chest)
As for most erotica that is full of romance feels it is portraying a fantasy for the readers where they can loose themselves. The First Person format of the book helps immensely with getting the reader to this place - believing them to be on the receiving end of Christian Grey.
As for your comment on having the sequels to be 'juicier', in my perspective as a reader of the FSOG series, I believe EL James didn't accomplish this, all the sex scenes were all the same if in different settings - never did Anastasia take the lead (portraying her as passive submissive) made my blood boil with anger.
I'm not sure what other scenarios that could be written for the erotica audience but prospective writers will find a way even if it might be niche.