SB 4.11.21

āyuṣo ’pacayaṁ jantos
 tathaivopacayaṁ vibhuḥ
ubhābhyāṁ rahitaḥ sva-stho
 duḥsthasya vidadhāty asau
Synonyms: 
āyuṣaḥ — of duration of life; apacayam — diminution; jantoḥ — of the living entities; tathā — similarly; eva — also; upacayam — increase; vibhuḥ — the Supreme Personality of Godhead; ubhābhyām — from both of them; rahitaḥ — free; sva-sthaḥ — always situated in His transcendental position; duḥsthasya — of the living entities under the laws of karma; vidadhāti — awards; asau — He.
Translation: 
The Supreme Personality of Godhead, Viṣṇu, is all-powerful, and He awards the results of one’s fruitive activities. Thus, although one living entity’s duration of life is very small whereas that of another is very great, He is always in His transcendental position, and there is no question of lessening or increasing His duration of life.
Purport: 

Both the mosquito and Lord Brahmā are living entities in the material world; both are minute sparks and are part of the Supreme Lord. The very short duration of the life of the mosquito and the very long lifetime of Lord Brahmā are both awarded by the Supreme Personality of Godhead according to the results of their karma. But in the Brahma-saṁhitā we find it said, karmāṇi nirdahati: the Lord diminishes or vanquishes the reactions of devotees. The same fact is explained in Bhagavad-gītā. Yajñārthāt karmaṇo ’nyatra: one should perform karma only for the purpose of satisfying the Supreme Lord; otherwise one is bound by the action and reaction of karma. Under the laws of karma a living entity wanders within the universe under the rule of eternal time, and sometimes he becomes a mosquito and sometimes Lord Brahmā. To a sane man this business is not very fruitful. Bhagavad-gītā (9.25) gives a warning to the living entities: yānti deva-vratā devān — those who are addicted to the worship of the demigods go to the planets of the demigods, and those who are addicted to worship of the Pitās, forefathers, go to the Pitās. Those who are inclined to material activities remain in the material sphere. But persons who engage in devotional service reach the abode of the Supreme Personality of Godhead, where there is neither birth nor death nor different varieties of life under the influence of the law of karma. The best interest of the living entity is to engage himself in devotional service and go back home, back to Godhead. Śrīla Bhaktivinoda Ṭhākura advised: “My friend, you are being washed away in material nature’s waves of time. Please try to understand that you are the eternal servant of the Lord. Then everything will stop, and you will be eternally happy.”