Reverse charge mechanish under gst

1.Under GST Law and Rules,  any time limit for amount paid under RCM  in Form GST-3 B,   if not taken as input tax credit . If Yes, then how much.

Problem,is one of my client has paid GST  tax Under RCM during August,2017, But not able to take Input Tax Credit  till February,2019. Is he be able to take ITC in March,2019. which was Paid Under RCM in August 2017.

2.If any error In GST-3 B, input tax credit taken in  Table 4 A (4), instead Of Table 4 A (3).How to rectify. 

Thanks in advance.