I watch this new trend to launch a beta signup page before the actual launch of the product and I wonder if they follow any rule regarding how long is this period.
Is simple until the product isn't ready or is some marketing behind? Do they wait until they have a number of signups? Is there an optimal fixed duration?
This can be really useful for working out if you are missing key features, or for getting ideas for future direction of the product.
Maybe the best use of a beta/coming-soon page though is to try and get an idea of the market size. Pretty much the first thing I do when I have a new product idea is to slap up a basic signup page, and spend a bit of money on Adwords to get an idea of whether this is something people want or not.
This also lets you get a feel for how much customers will cost you to acquire (through Adwords at least) which can feed into your business/cash-flow planning.
It also allows you to get your page indexed and ranked by Google, meaning you'll have an easier time of it when you start trying to drive organic traffic to your site when you launch.