The pERK antibody that most people use is this one from Cell Signaling: https://www.cellsignal.com/products/primary-antibodies/phospho-p44-42-mapk-erk1-2-thr202-tyr204-antibody/9101
This is an excellent antibody and gives good staining if there is real signal. I've used at 1:500 in free floating spinal cord sections. @zhzhj131421 and @zhisrich have both gotten beautiful staining with this ab in this way. @janehartung also has experience pERK staining with this antibody.
Best time point is dependent on what model or stimulus you're using. See the other discussion here about neuropathic models:
In general though, after a stimulus, such as capsaicin or other noxious stimulus, the pERK signal is very rapid, peaking at about 5 mins. This is in contrast to IEGs such as Fos, which usually peak around 1-2 hours. See the review by Gao and @rurongji : https://www.ncbi.nlm.nih.gov/pubmed/19898681