Without any extra experience, nor Rare Candy :

Ralph	: 237 exp	~ 0:54, with 1 Supersonic
Gordon	: 111 exp	~ 0:46
Peter	: 239 exp	~ 1:11
Russell	: 329 exp	~ 0:59 w/ WG | 1:38 w/out WG
Bill 	: 290 exp
Benny	: 622 exp

44 	to reach 6 	before Rival 1 	(Chikorita)
34	to reach 7	before Mikey 	(Pidgey + Rattata)
42	to reach 8	before Abe 	(Spearow)
36	to reach 9	before Rod	(Pidgey x2)
13	to reach 10	before Falkner	(Pidgey + Pidgeotto)
127	to reach 12	before Rocket 1	(Slowpoke Well, Rattata x2)
415	to reach 13	before Rocket 2 (Slowpoke Well, Zubat + Ekans)	---	Russel + 125 // Bill + 164 // Ralph + Wooper + 67 (+100 for 19)
215	to reach 14 	before Zubat x2 (Rocket 3, Slowpoke Well)
9	to reach 14	before Koffing	(Rocket 4, Slowpoke Well)
286	to reach 18	before Rival 2
777	to reach 19	before Bayleef	(Rival 2)