Campaign: 10–15 hours (Normal). 100% / All Intel / Elite: 25–30 hours.
Multiplayer (Ad-Hoc / Infrastructure): Infinite. This was the killer app for PSP online (2006–2012). 16-player matches, clans, tournaments, custom maps. Community keeps it alive via XLink Kai / Pro Online (custom DNS) on CFW PSP/Vita.
Verdict: Short campaign, legendary multiplayer longevity. If you only play SP: short. If you played MP: hundreds of hours.