Sunnah Prayers: What Are Their Virtues and Times?
Question

I would like to know whether you wait for the time of the prayer before praying any sunnah prayer which comes before it; or whether these rak`ahs are prayed before the time for the prayer enters. If so; how long before a prayer can you pray the sunnah prayers which come before it? For example, could I pray the 4 sunnahs which come before ‘Asr 3 hours before the time for ‘Asr enters? Or do I wait for the time of ‘Asr to enter and then pray the 4 Sunnah rah’ahs after the time for ‘Asr has entered and follow them immediately by the obligatory prayer?
Praise be to Allah.Sunnah prayers before and after obligatory prayers
The regular Sunnah prayers fall into two categories: 

Sunnahs which are done before the obligatory prayer – two rak’ahs before Fajr and four before Zhuhr . 
Sunnahs which are done after the obligatory prayer – two rak’ahs after Zhuhr, two rak’ahs after Maghrib and two rak’ahs after ‘Isha. 

Virtues of sunnah prayers
Concerning the virtues of sunnah prayers, Prophet Muhammad (peace and blessings of Allah be upon him) said: “Whoever prays twelve rak’ahs during the night and day, a house will be built for him in Paradise: four before Zhuhr and two after, two rak’ahs after Maghrib, two rak’ahs after ‘Isha and two rak’ahs before Fajr prayer.” (Narrated by al-Tirmidhi, no. 380; classed as sahih by al-Albani in Sahih al-Jami’, 6362) 
Time of sunnah prayers
The time for a Sunnah prayer that comes before an obligatory prayer is from the beginning of the time for that prayer, until the iqamah. 
The time for a Sunnah prayer that comes after an obligatory prayer is from after the salam at the end of the obligatory prayer until the end of the time for that prayer. 
Ibn Qudamah (may Allah have mercy on him) said: “The time for every Sunnah prayer that comes before an obligatory prayer is from the beginning of the time for that prayer, until the prayer is done, and the time for every Sunnah prayer that comes after an obligatory prayer is from after the prayer is done until the end of the time for that prayer.” (Al-Mughni, 2/544)
Based on this, it is not correct to pray the Sunnah prayer before the time for the obligatory prayer begins, rather you should wait until the time for that prayer comes, then pray it. 
For Jumuah sunnah prayer, please see this answer: 181043
And Allah knows best.