Different pokémon available on Sehto have different lifespans. Ordered by length of average lifespan, descending:
Immortal
These pokémon simply will not die from old age. They can still be killed by regular means, though, providing you can pull it off:
Mortal, approaching the lifespans of trees (500y+)
Mortal, exceeding the lifespans of whales (251y-500y+)
Mortal, sharing the lifespans of whales (131y-250y)
250y: Abra / Kadabra / Alakazam
240y: Magikarp / Gyarados
240y: Snorlax
230y: Paras / Parasect
220y: Bulbasaur / Ivysaur / Venusaur
170y: Lickitung
160y: Natu / Xatu
150y: Qwilfish
-
Mortal, exceeding human lifespans (81y-130y)
130y: Growlithe / Arcanine
130y: Chansey / Blissey
125y: Phanpy / Donphan
110y: Charmander / Charmeleon / Charizard
108y: Mareep / Flaaffy / Ampharos
100y: Smoochum / Jynx
97y: Swinub / Piloswine
97y: Magby / Magmar
95y: Squirtle / Wartortle / Blastoise
93y: Lapras
90y: Kangaskhan
85y: Oddish / Gloom / Vileplume / Bellossom
-
83y: Elekid / Electabuzz
82y: Teddiursa / Ursaring
81y: Skarmory
Mortal, approximately matching human lifespans (61y-80y)
-
80y: Mantine
78y: Sentret / Furret
78y: Machop / Machoke / Machamp
77y: Smeargle
75y: Pidgey / Pidgeotto / Pidgeot
-
72y: Sudowoodo
70y: Koffing / Weezing
69y: Zubat / Golbat / Crobat
68y: Sandshrew / Sandslash
67y: Tangela
65y: Girafarig
65y: Tyrogue / Hitmonlee / Hitmonchan / Hitmontop
64y: Cleffa / Clefairy / Clefable
64y: Igglybuff / Jigglypuff / Wigglytuff
63y: Pichu / Pikachu / Raichu
62y: Houndour / Houndoom
62y: Vulpix / Ninetales
62y: Nidoran / Nidorina / Nidorino / Nidoqueen / Nidoking
62y: Scyther / Scizor
61y: Poliwag / Poliwhirl / Poliwrath / Politoed
61y: Ekans / Arbok
Mortal, long-lived (36y-60y)
60y: Ponyta / Rapidash
60y: Seel / Dewgong
60y: Shellder / Cloyster
58y: Rhyhorn / Rhydon
38y: Totodile / Croconaw / Feraligatr
55y: Pineco / Forretress
52y: Togepi / Togetic
51y: Delibird
50y: Bellsprout / Weepinbell / Victreebel
50y: Dratini / Dragonair / Dragonite
48y: Kabuto / Kabutops
48y: Drowzee / Hypno
45y: Tentacool / Tentacruel
42y: Snubbull / Granbull
42y: Heracross
41y: Chikorita / Bayleef / Meganium
40y: Meowth / Persian
40y: Corsola
-
38y: Horsea / Seadra / Kingdra
38y: Omanyte / Omastar
37y: Cyndaquil / Quilava / Typhlosion
36y: Psyduck / Golduck
36y: Farfetch'd
36y: Pinsir
Mortal, medium average lifespan (16y-35y)
Mortal, short-lived (up to 15y)
15y: Eevee / Vaporeon / Jolteon / Flareon / Espeon / Umbreon
15y: Spinarak / Ariados
14y: Doduo / Dodrio
14y: Grimer / Muk
13y: Krabby / Kingler
13y: Staryu / Starmie
12y: Exeggcute / Exeggutor
12y: Aipom
12y: Sunkern / Sunflora
11y: Caterpie / Metapod / Butterfree
10y: Dunsparce
-
9y: Chinchou / Lanturn
9y: Hoppip / Skiploom / Jumpluff
8y: Weedle / Kakuna / Beedrill
8y: Yanma
8y: Ledyba / Ledian
7y: Ditto