You need to be prepared at all times to explain your idea. The amount of time you have to explain it will depend on a number of factors, not least of which is the amount of 'face-time' your stakeholder will give you. Here are three formats you should have prepared for selling your idea depending on how much time you can get:
- The Tweet version: A tweet (as delivered via twitter) can be a maximum of 140 characters. The challenge is can you describe your idea in 140 characters or less. Samuel Johnson (or Mark Twain or Winston Churchill depending on who you believe) said "I did not have time to write you a short letter, so I wrote you a long one instead". This version is the most challenging of all. You really need to be brutal and pare your idea down to just the key facts.
- The Cocktail Party version: This is a variant on the elevator pitch. Can you describe you idea in 100 - 150 words or a maximum of one minute of talking (talking fast doesn't count). Again you need to focus on the bare essentials but here you have a bit more leeway to focus on the business benefits.
- The Real-Deal version (with supporting takeaway): So you twittered your idea, you met some guy at a cocktail party (or in the elevator) to entice him a bit more and you finally got invited to present your idea. The presentation is the real-deal because this is really your chance to stand up and sell (and hopefully clinch the deal). Don't, therefore, screw-up by preparing an overly busy presentation with slides full of tightly packed text (remember PowerPoint bullets kill interest like real bullets kill people). Suppose you have "an hour" to present. Aim for a presentation that can be done in 30 minutes allowing for 15 minutes of questions and five minutes or so either side for people to be late or have to leave early. No one can retain an interest for more than 50 minutes anyway so 30 is good. For some thoughts on presenting see here. I prefer not to follow rules like "one slide every two minutes". The important thing is to structure the presentation first (probably before opening up your favourite presentation software) then write it, then practice until it fits into 30 minutes. For an interesting alternative view on how big a presentation should be see here (a slide every 12 seconds maybe!). Finally, because you will inevitably have had to leave out some detail prepare a short (two to four pages) takeaway which explains your idea that you can leave behind for your audience to take-away. Make sure you include the tweet as the "management summary". You never know, your stakeholder may tweet it herself giving you a bit more publicity!