Albert Barnes offers the following guidelines:-
2. It is not easy, perhaps not possible, to maintain a life of piety without regular habits of secret devotion.
3. The morning, when we have experienced God’s protecting care, when the mind is fresh, and the thoughts are as yet clear and unoccupied with the world, when we go forth to the duties, trials, and temptations of the day; and the evening, when we have again experienced his goodness, and are about to commit ourselves to his protecting care, and when we need his pardoning mercy for the errors and follies of the day, seem to be times which commend themselves to all as appropriate seasons for private devotion.
4. Every person will also find other times when private prayer will be needful, and when he will be inclined to it. In affliction, in perplexity, in moments of despondency, in danger, and want, and disappointment, and in the loss of friends, we shall feel the propriety of drawing near to God, and of pouring out the heart before him.
5. Besides this, every Christian is probably conscious of times when he feels peculiarly inclined to pray; he feels just like praying; he has a spirit of supplication; and nothing but prayer will meet the instinctive desires of his bosom…Such seasons should be improved; they are the “spring times” of our piety; and we should expand every sail, that we may be “filled with all the fullness of God.” They are happy, blessed moments of our life; and then devotion is sweetest and most pure; and then the soul knows what it is to have fellowship with the Father and with his Son Jesus Christ, 1 John 1:3.
6. In addition to all this, Christians may be in the habit of praying to God without the formality of retirement. God locks upon the heart; and the heart may pour forth its secret desires to Him even when in business, when conversing with a friend, when walking, when alone, and when in society.
Thus the Christian may live a life of prayer; and it shall be one of the characteristics of his life that he prays! By this he shall be known; and in this he shall learn the way to possess peace in religion.
Notes on the New Testament: Romans 12:12
The life of prayer