Since Grado headphones are supra-aural (rest on the ears), it isn't a question of if you ears will become tender, it is a question of how long before your ears become tender. This varies a lot from one person to the next.
Bending the headband out such that the pressure exerted on your ears is lessened helps as lot, as does pad selection.
For me, a properly adjusted headband along with flat pads allows me to listen for upwards of 8 hours comfortably before I need to give my ears a rest. I can't go nearly as long with bowl pads - maybe 3 or 4 hours before needing a rest.
As others have mentioned, the headphile pads are an option. But those pads transform Grado headphones into something they weren't intended to be (only exception being the GS1000s, since they are indeed intended to be circumaural phones). As such, the sound signature changes, and you may not like that change in sound. Again, this opinion varies from person to person also.