It's nice that you're offering a longer trial period for people who specially request it, but what they're saying (which I 100% agree with, and am also not in the market for accounting software, to be clear) is that you're likely to lose out significantly with such a short period. My guess is if you offered a 3-month trial period you'd get over 10 times as many signups.
Just my 2 cents.
The only way the 14-day period seems like a good idea to me from a (short-sighted) business standpoint is if there are significant problems with your software that don't reveal themselves until books for the month are done. That way you can get some conversions (followed by cancellations when they run into those shortcomings) from people who wouldn't convert if you offered a longer period.
If you're worried about large customers taking advantage and becoming difficult to service for such a long period, another model is to offer a free tier with up to X data, Y records, etc. That way you'll be appealing to smaller businesses for whom this is roughly 3 months of accounting services, and perhaps less so to larger businesses who usually take much longer to convert anyway.