Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

 

There is no preferred time for offering istikharah prayer. It is only disliked to offer it at certain times such as from fajr until the sunrise, immediately before Zhuhr prayer, after `Asr until the sun sets.

 

This is in case the reason for praying istikharah is not prompt. Otherwise, you can pray it anytime. Still you can choose times when du`aa’ is mostly accepted such as the last sixth of the night but there is not particular evidence to support this.

 

Allah Almighty knows best.