Much as I like numbers and use them in evaluating my own business, the most important thing I see in your information is that there are 8 pizza joints in the market.
A market where 8 pizza places can survive is one where a good operator will do fine. There is enough business. I never assume that the business will be split evenly. In fact, I would assume it is not. It may well be possible to get 25% or even 35% of the business rather than half that. It may also be possible to beat the averages by attracting customers that don’t order pizza for delivery since most shop deliver a lousy product or to increase the $$ by justifying higher prices with quality or interesting add-on sales.
I would not suggest going after the strengths of the nationals though. The cost advantages they possess make going after the low-price/volume model a tough row to hoe. The good news is that, simply put, the nationals make lousy pizza and offering something consistant, interesting and priced fairly with good service often does well.
In a race with a bear, I do not have to be faster than the bear, I just have to be faster than some other guy.
But, to answer your question; How long? Much depends on how you have set yourself up initially. For example, if you have leases on your equipment and computers rather than purchasing them outright to begin with, you have raised the monthly “nut” and changed the break even formula. How much time you spend working yourself will make a difference as will your opening marketing budget. If you are waiting for folks to find you, it could take a while, if you have 10K to spend telling them you are there it will go a lot faster. So… there is no one right answer to your question.