Feature shelf life
So I don’t have a formula for quick and accurate estimates. Just a lot of hard work. Still, here’s a tip for free: anyone asking for a firm delivery date is inherently assuming BDUF. Once you know that, you know where to start your answer.
No disagreements with Jacob here, just want to add shelf life to the reasons your business partner will ask for software delivery dates. There are situations when you will be asked for a delivery date in order to get some sense of the freshness of the feature. This date is actually secondary to the primary goal (design win, show demo, competitive trump, industry announcement, etc.).
There is some absurdity in looking at software features this way. You and I might rightly assume that a brilliant feature is justification in its own right. Rationally, a feature’s implementation velocity should just be a stakeholder decision of how much resource they are willing to commit to its completion. Yet, your biz partner may conclude that if the feature can’t be completed in a particular time frame, then its value goes to zero.
Dealing with this is one is tricky, so use all of the communication techniques and energy that Jacob recommends. The best luck I’ve had with this scenario is to defer the answer to the start of the next development iteration. This forces everyone to prioritize the feature in the context of the backlog. If it really is the highest priority thing to do, then we’re in a position to go deep and scope it right away.