15 does seem like quite a lot, I can understand getting some form fatigure.
Depending on what kind of information you are collecting, I would try getting it in some other way.
1) Personal or company data- There are a lot of enrichment options our there that will integrate with Marketo/your CRM that will allow you to lookup info about the company/person based on a combination of IP address and email. Using those to get your revenue, employee count, etc can help a lot. Some of them are even decent at providing personal information.
2) Buying stage/ needs- Instead of asking the prospect where they are at, and what they are looking for, is there any way you can infer this info from what they are looking at? With a lot of assents on our site they fit in different places in the buying cycle, and we use the customer's natural browsing habits to infer what stage they are at, and whats needs they are trying to fill. I've also heard a lot of success with basic "decision trees" naturally in certain parts of your webiste. For example, clicking on the "resource center" brings up a "Are you a national brand or a local franchisee?" overlay with 2 big buttons, and they have to simply click one to continue. While a lot of these can get annoying, one or two strategically placed can inform you a lot about what the customer is looking for.
Personally, once a person fills out a form once for our site, I try and make it as easy as possible for that prospect to continue engaging with us. Try to minimize form fillouts for "known" prospects (you can do dynamic content with Marketo, segmenting off "known" leads versus "anonymous" leads and feading them different content). We try and use auto-filled out forms, and segmentation to remove the barriers to our content as much as possible, without compromizsing our tracking abilities.
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