It is possible to configure a different lifespan for access tokens on a per client basis. In Keycloak admin console go to a client settings page and expand the “Advanced Settings” section.
This screenshot is taken from Keycloak 4.8.1.Final.
EDIT: Be aware that is override is applied to Authorization Code Flow only. The access token lifespan for Implicit Flow can still (Keycloak 7.0.0) be set on realm level only!
EDIT: Since Keycloak 10.0.0 it is also possible to override session idle and session max timeout per client.