[smallerfish]

Start by listing out the features in a spreadsheet. For each feature, think through it and list out the stories, one per row. Create a section per feature in the spreadsheet (i.e. put a line under each group of rows).

For each story, add an initial estimate (in terms of developer-days). This is your "low" estimate. Now in a second column, add a "high" (potential-but-reasonable "worst case") estimate. If you're looking at more than 10-15 days for either column for a story you should probably break the story up some more.

Now add a 3rd/4th column, which are the low/high estimates multiplied by 1.3 ("fudged low" / "fudged high"). Total up all stories per feature in a row at the bottom of each feature's section. Divide by team size, divide by business days, round up to nearest integer, and you have your calendar weeks for each feature.

When sales/marketing ask you for estimates, you then respond "between X and Y calendar weeks from [completion of previous feature]". Just be aware that they will hear X, so make sure Y is very clearly included in every communication where the dates are being discussed. If "previous feature" slips, make sure to communicate clearly that "next feature" has also pushed back by however many weeks. You'll be tempted to, but don't be optimistic with progress reports or estimates of where you are in the range - undersell and over-deliver, and you'll keep more allies on the business side.