Duration of salat

The duration of the salat should be according to the Sunnah. In Fajr and Zuhr salat, it is Sunnah in the first two rak’ats to recite any Surah from Surah Al-Hujuraat to Surah Al-Burooj (inclusive). In Asr and Isha salat, it is Sunnah in the first two rak’ats to recite any Surah from Surah At-Taariq to Surah Al-Bayyina (inclusive). And in Maghrib salat, it is Sunnah in the first two rak’ats to recite any Surah from Surah Zalzalah to the last Surah of the Quran. This applies to an imam (one who leads others in the salaat) and also to a munfarid (one who offers his salaat individually).

(و) يسن (في الحضر) لإمام ومنفرد، ذكره الحلبي، والناس عنه غافلون (طوال المفصل) من الحجرات إلى آخر البروج (في الفجر والظهر، و) منها إلى آخر – لم يكن – (أوساطه في العصر والعشاء، و) باقيه (قصاره في المغرب) أي في كل ركعة سورة مما ذكر، ذكره الحلبي ( الدر المختار: ج 2 ص 317 ط دار المعرفة )

( رد المحتار: ج 2 ص 319-320 ط دار المعرفة )

As for the tasbeeh in the ruku’ and sujood, an imam should only say it three times. One following the imam should say it till the imam gets up from the ruku’ or sujood. As for a munfarid (one who offers salaat individually), if he says it more than three times, it is better.

ولو زاد على الثلاث فهو أفضل ؛ لأن قوله وذلك أدناه دليل استحباب الزيادة ، وهذا إذا كان منفردا فإن كان مقتديا يسبح إلى أن يرفع الإمام رأسه ، وأما إذا كان إماما فينبغي أن يسبح ثلاثا ولا يطول على القوم لما روينا من الأحاديث ولأن التطويل سبب التنفير وذلك مكروه ( بدائع الصنائع: ج 1 ص 309 ط دار الفكر )

You should not deprive yourself of the reward of offering fard salaat in the masjid. Keep three things in mind: 1) The reward of offering Fard salaat in the masjid is 25 or 27 times more than offering it at home. Abu Hurairah (radiyallahu anhu) narrates that Rasulullah (sallallahu alaihi wa sallam) said “The salaat of a person in congregation is 25 times more (rewarding) than his salaat at home or in the market…”  (Sahih Bukhari, hadith 611)  2) A person cannot get closer to Allah Ta’ala more than by following the Sunnah of Rasulullah (sallallaahu alaihi wa sallam); and the Sunnah of Rasulullah (sallallaahu alaihi wa sallam) is to offer the fard salaat in the masjid. 3) Rasulullah (sallallaahu alaihi wa sallam) has ordered anyone who leads others in salaat to keep the salaat short. Abu Hurairah (radiyallaahu anhu) narrates that Rasulullah (sallallaahu alaihi wa sallam) said “When one of you leads others in salaat, he should make it short, because among them will be the young, the elderly and the sick. And when one offers his salaat by himself, then he may offer his salaat howsoever he wishes.” (Saheeh Muslim, hadith 715). Anas Ibn Maalik (radiyallaahu anhu) says, “I have never offered salaat behind any imam with shorter yet more complete salaat than that of Rasulullah (sallallaahu alaihi wa sallam).” (Sahih Muslim, hadith 721).

As for the length of the dua, if after the fard salaat there is Sunnah salaat, then one should only sit for approximately the period of time it takes to say:

اللّهُمَّ اَنْتَ السَّلامُ وَمِنْكَ السَّلَامُ  وَإِلَيْكَ يَعُوْدُ السَّلَامُ تَبَارَكْتَ يَا ذَا الْجَلَالِ وَالْإِكْرَام

Then he should stand up to offer the Sunnah salaat. At the end of the salaat, however, he may make dua for however long he wants.

( انظر: مراقي الفلاح وحاشية الطحطاوي: ص 311-312 ، 316-317 ط العلمية )

As for your istikhaara on this matter, well it is as a result of your istikhaara that you sent your query to us to find out the ruling according to the Shariah. Otherwise, istikhaara is for seeking khair (good) i.e. guidance from Allah Ta’ala in matters that are not already established in the Shariah, such as, in the case of one on whom Hajj is fard, which flight to take when going for Hajj — and not whether he should go for Hajj or not.

وقالوا الاستخارة في الحج ونحوه تحمل على تعيين الوقت ( رد المحتار: ج 2 ص 570 ط دار المعرفة )