For how long do I have to promote my app?

There is no common or specific strategy. Let's just consider two types of promotion, which generally suit two types of applications: for new and for those apps which have already been on the market.

If your application is completely new start your promotion using the Package ID installs. This type of promotion will help you rank for more keywords and you can then promote with keyword installs. For start, you can promote your application with 200-500 daily installs, and then increase the amount to 700-1000. Keep increasing the number of daily installs.

If your application is already on the market - it has its own specific keywords, and you can create a campaign for 3-5 keywords and promote it within one week with 700-1000 installs per day. Also, it is very important to remember that the effect will not last forever and if you notice that the rank is falling - you should also order the installs for visibility of your app in the TOP.