Ahh, I had assumed you were talking about a brand new application/service. If, as you mentioned in a reply to another comment, you're just exposing an existing web service to mobile phones then simply providing a mobile alternative to that service gets you a large percentage of your likely user base for a minimal effort.
If you're asking when 100% of the phones in your possible market will have browsers on them that are HTML5 aware/capable, I couldn't begin to guess. I don't think that will happen soon, but it's certainly happening at a faster rate than I ever thought it would be. This is the kind of question that can have a paralyzing effect however. If you want to provide a mobile gateway to your existing service you should go for it and you should implement to target the phones you expect the majority of your users to be using.
It was pointed out earlier, but it bears repeating, plan for what's out there right now. HTML5 has a lot of momentum in the browser space, but that doesn't mean it will be adopted in a uniform manner. Limit your scope if you have to, something is better than nothing, but right now the state of mobile browsers is in flux and I wouldn't recommend depending on 3rd parties to create a desirable environment for you.
