In Tolkien's sagas, life-spans of "Half-Elven" progeny do not "decrease" from that of the mortal parent - they increase significantly. This is due quite obviously to the 50% influx of the true-immortal blood of the Elven parent. Eldarion would thus not live "less long" than his mostly mortal father - as his mother Arwen was a true immortal-High-Elf of Noble lineage.
Indeed, Arwen's lineage actually has more immortal Elvish blood than her father Elrond and uncle Elros. She is also only one generation more removed from her great grandfather Dior - who was 1/4 Maiar-god. Elros and Elrond are 3/8th human (by their human Grandfather Tuor; and 1/2 human Grandfather Dior). Arwen is only "3/16ths" Edain - so in theory and in mathematical fact: her true-immortal DNA is far stronger and more pure.
Aragorn also is not a mere "Dúnedain", but in fact he has some "Elvish Blood" as a direct descendant of Elendil and Elros. The Lords of Andúnië were all descendants of Silmariën - the firstborn-and-daughter of the 4th King-descendant of Elros (and arguably, the true-heir to the throne). So too does Aragorn have "a trace" of immortal Maiar-blood. This does not include any additional later influx of the "Blood of Elros" by Andúnië-marriages to more female descendants of Elros. There should/would have likely been several due to the close relationship and alliance between the Kings of Númenor and the Lords of Andúnië. (It can be speculated that in later generations, the Lords of Andúnië actually had more Blood of Elros in their line than the later Kings did, due to the majority of children not becoming "king" - and marrying then into the line of the Andúnië.)
We do have several, several examples of the "lifespan" of Half-Elven children like Eldarion (notwithstanding "choices" about ending one's life early and "choosing to die", as apparently most royal half-elves "choosing" a mortal life do). Dior, Eärendil, Lúthien, Elwing, Eluréd, Elurín, Elladan, Elrohir and Arwen were all "half-elves" - and were all, essentially, Immortal - living in many cases thousands of years. Arwen later "chose a mortal life" and willed herself to die (of grief) at the age of 2,901. Elros too "chose to die" on his 500th Birthday.
Elros, like Arwen, had sons with mortal humans - although as mentioned: Arwen's blood was more-pure with immortal non-human lineage. Also, Arwen's mate Aragorn had trace Evlish blood as well - unlike the merely-human wife of Elros. Eldarion, thus, should live at Least as long as Elros's son Vardamir - who "decided" to die at 471 years old. Any other grandsons or great grandsons of Elros that are recorded all lived past the age of 400. The likely-greater life spans of all other descendants are not recorded, including those not burdened by the stresses of leadership, and all females with universally greater average lifespans than males.
The potential lifespan of Eldarion should be at least that of the less-immortal-blooded Vardamir: 471 years (or longer).