A few days or so I think... The big concern is fevers or whether there's streaking from the injection site (infection)... or swelling that gets worse instead of better.
Yeah, a few days. At least with my kids.
Ds2 had redness and swelling and a bruise after his 4 month shots. His first ones he didn't have anything. The nurse the second time around didn't pinch up his skin like they normally do so it lead to a bruise on all 3 injection sites, only one swelled up and was red for 3-4 days.