I went to the Sacrament of Reconciliation on Thursday morning, all went well and was given a penance. I forgot about this and only did it this (Wednesday) afternoon. Is this still valid completion of the penance?
There is no technical time frame in which the penance must be said/done.
That being said, its important to note that the forgetting to say/do one’s assigned penance does not invalidate the sacrament. All that is required for validity is that all mortal sins be confessed, the penitent be sorry for their sins, and proper absolution be given by the priest. If someone forgets to do their penance then they are still forgiven. If someone intentionally refuses to do their penance then it calls into question if they were truly sorry for their sins and thus calls into question the validity of their confession.
In your case you simply forgot, so you did it when you remembered. You’re a-ok.