Sometimes the Imaam recites in a lengthy manner during inaudible prayers and I finish the recitation of Al-Faatihah and another Surah, so what should I do then? Is it permissible to recite Surahs of the Quran, after reciting Al-Faatihah in the last two Rak‘ahs (units of prayer) of the prayer, in case the Imaam prolongs the recitation?
May Allaah Almighty reward you.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If one being led in an inaudible prayer finishes the recitation of Al-Fatihah in addition to another Surah and the Imaam still does not bow, then he should continue reciting the Quran, regardless if what he recites comprises a Surah or less than that.

If he does not recite the Quran, he may make Thikr (remembrance of Allaah). But he should in no way stop reciting Quran or making Thikr, in order not to be overtaken by insinuations or thoughts about matters that are irrelevant to prayer.

According to the preponderant opinion on this issue, it is permissible for one led in prayer to recite Surahs other than Al-Faatihah in the last two Rak‘ahs of the four-Rak‘ah prayer and the third Rak‘ah of the three-Rak‘ah prayer.

Allaah Knows best.