The Orthodox Christian saint Theophan the recluse (1815-1894 CE) gave the following advice ‘for one who is on the path of a God-pleasing life.’ By prayer rule, he means a self-imposed regime. For more about Theophan, see this post.

You ask about a prayer rule. Yes, it is good to have a prayer rule on account of our weakness so that on the one hand we do not give in to laziness, and on the other hand we restrain our enthusiasm to its proper measure. The greatest practitioners of prayer kept a prayer rule. They would always begin with established prayers, and if during the course of these a prayer started on its own, they would put aside the others and pray that prayer. If this is what the great practitioners of prayer did, all the more reason for us to do so. Without established prayers, we would not know how to pray at all. Without them, we would be left entirely without prayer.

However, one does not have to do many prayers. It is better to perform a small number of prayers properly than to hurry through a large number of prayers, because it is difficult to maintain the heat of prayerful zeal when they are performed to excess.

I would consider the morning and evening prayers as set out in the prayer books to be entirely sufficient for you. Just try each time to carry them out with full attention and corresponding feelings. To be more successful at this, spend a little of your free time at reading over all the prayers separately. Think them over and feel them, so that when you recite them at your prayer rule, you will know the holy thoughts and feelings that are contained in them. Prayer does not mean that we just recite prayers, but that we assimilate their content within ourselves, and pronounce them as if they came from our minds and hearts.