There are two times for praying Asr prayer.
The normal time begins from when the shade of an erected object is equivalent to its length. The evidence for this is the hadeeth of Jibreel, peace be upon him when he prayed with the Prophet salla Allahu alaihi wa sallam telling him the times of prayers.
The second time is for emergencies and this is extended up to just before sunset. The evidence for this is the hadeeth were the Prophet salla Allahu alaihi wa sallam says that whoever prays one Rak’ah before sunset, he has prayed Asr.
As for deliberately delaying the prayer till the time is out, there is no Kad’ah for that and it wouldn’t be accepted if a person delays it for no legitimate reason. He is sinful and this is a major sin.