Killstreaks last 30% longer. This has no effect if it is used on classes with Betsy, Deadly Shurikens and Smoke bomb because these don't have expiration time but they can be used until ammo runs out (or last only a second until the Ninja gets teleported). Mercenary and Ninja can't use utility upgrades anyway, so it's impossible to have an upgrade like this.
However, Mercenaries can use Ghost Weapon which would be very powerful with this upgrade, and if you get it in the Slot Machine you can equip Mercenary with it. Also, Dex the unique Mercenary has that killstreak. He also can use utility upgrades by default because his upgrade order is unique but he doesn't have Long Lasting among his normal upgrades. Other unique heroes, Jyn and Calamity have this upgrade.