John Gill Commentary Jeremiah 29:12

"And ye shall call upon me, and ye shall go and pray unto me, and I will hearken unto you." — Jeremiah 29:12 (ASV)

Then shall you call upon me
When the expected end is about to be given; when God intends and is about to bestow a mercy, he gives his people a spirit of prayer to ask for it; and even the promise of it is a considerable argument to encourage and engage more to pray for it:

and you shall go and pray unto me :
walk in my ways; so Jarchi, Kimchi, and Ben Melech; or rather you shall go into your private closets, or into those public places where prayer was wont to be made, and there put up your petitions; or it may be the meaning is, that they should continue praying unto him; should pray without ceasing, until they enjoyed the blessing, and had the expected end given them:

and I will hearken unto you :
God is a God hearing prayer; he listens to the requests of his people, and answers them in his own time and way; which is no small encouragement to pray unto him.