I believe the R21 is the high risk NIH grant proposal. These usually last I think for three years if I'm not mistaken. I don't think you will be listed as a co-PI not as a post-doc I don't think but you should ask your supervisor in any case. Besides if it is an R21 grant that you're helping write then most likely there is funding allocated within the grant for post-doc positions. Usually the PIs and co-PIs are individuals with permanent faculty positions and occasionally senior research staff members. But for anyone else out there who may or may not more about this feel free to enlighten me.