Your pet will not gain XP if it is a 0 health, 0 hunger, is not equipped, or is out of range and "lost." Make sure to watch its health and hunger as you fight. As soon as the health hits 0 your pet will die and become sealed; as soon as the hunger hits 0 your pet will stop moving. If your pet is not equipped, you can equip it by opening your inventory, finding your pet's icon, and double-clicking it. If your pet is "lost" or has gotten left behind, you can either unequip and re-equip it, or summon it by using the Pet summon command. If you have checked all of the above possibilities and your pet is still not gaining XP, you may need to change monsters or monster types. Fighting blue-named monsters--monsters that are 6 or more levels below you--will not earn your pet XP.

Like your XP, the XP your pet needs to gain each new level continually increases. Fortunately, pet XP does not go up at as steep a rate as our characters' XP does. If you go through your levels fighting mostly with normal attacks, you will find that your pet levels up almost 3 times faster than you do! This is a good thing because higher level pets can learn more skills to help you--but remember that if the pet gets above your level it will start eating huge amounts. So be careful not to let that happen.

At the moment, the highest pet level is 100. The pets will get experience just like P2 pets in a future patch, it is not known when.

Look at this chart for the amount of pet XP needed at each level. XP amounts were taken from this chart and from a list provided by Graut.

Level	XP amount		Level	XP amount		Level	XP amount		Level	XP amount
1	654 (654)		26	5,639 (61,022)		51	48,629 (581,672)		76	419,338 (5,071,360)
2	712 (1,366)		27	6,147 (67,169)		52	53,006 (634,678)		77	457,079 (5,528,439)
3	777 (2,143)		28	6,700 (73,869)		53	57,777 (692,455)		78	498,216 (6,026,655)
4	846 (2,989)		29	7,303 (81,172)		54	62,977 (755,432)		79	543,055 (6,569,710)
5	923 (3,912)		30	7,960 (89,132)		55	68,644 (824,076)		80	591,931 (7,161,641)
6	1,006 (4,918)		31	8,677 (97,809)		56	74,823 (898,899)		81	645,204 (7,806,845)
7	1,096 (6,014)		32	9,457 (107,266)		57	81,557 (980,456)		82	703,273 (8,510,118)
8	1,195 (7,209)		33	10,309 (117,575)		58	88,897 (1,069,353)		83	766,567 (9,276,685)
9	1,303 (8,512)		34	11,237 (128,812)		59	96,897 (1,166,250)		84	835,558 (10,112,243)
10	1,420 (9,932)		35	12,248 (141,060)		60	105,618 (1,271,868)		85	910,759 (11,023,002)
11	1,548 (11,480)		36	13,350 (154,410)		61	115,124 (1,386,992)		86	992,727 (12,015,729)
12	1,687 (13,167)		37	14,552 (168,962)		62	125,485 (1,512,477)		87	1,082,073 (19,097,802)
13	1,839 (15,006)		38	15,862 (184,824)		63	136,779 (1,649,256)		88	1,179,459 (14,277,261)
14	2,005 (17,011)		39	17,289 (202,113)		64	149,089 (1,798,345)		89	1,285,610 (15,562,871)
15	2,185 (19,196)		40	18,845 (220,958)		65	162,507 (1,960,852)		90	1,401,315 (16,964,186)
16	2,382 (21,578)		41	20,541 (241,499)		66	177,133 (2,137,985)		91	1,527,434 (18,491,620)
17	2,596 (24,174)		42	22,390 (263,889)		67	193,075 (2,331,060)		92	1,664,903 (20,156,523)
18	2,830 (27,004)		43	24,405 (288,294)		68	210,452 (2,541,512)		93	1,814,744 (21,971,267)
19	3,084 (30,088)		44	26,602 (314,896)		69	229,392 (2,770,904)		94	1,978,071 (23,949,338)
20	3,362 (33,450)		45	28,996 (343,892)		70	250,038 (3,020,942)		95	2,156,098 (26,105,436)
21	3,665 (37,115)		46	31,606 (376,498)		71	272,541 (3,293,483)		96	2,350,147 (28,455,583)
22	3,995 (41,110)		47	34,450 (409,948)		72	297,070 (3,590,553)		97	2,561,660 (31,017,243)
23	4,354 (45,464)		48	37,551 (447,499)		73	323,806 (3,914,359)		98	2,792,209 (33,809,452)
24	4,746 (50,210)		49	40,930 (488,429)		74	352,949 (4,267,308)		99	3,043,508 (36,852,960)
25	5,173 (55,383)		50	44,614 (533,043)		75	384,714 (4,652,022)		100	3,317,424 (40,170,384)

To check your pet's XP amount, equip the pet and then hit the hotkey O or ALT+O. That will open up the pet's status window for you. The XP number is listed under the pet's level, health, and hunger.

Here's a helpful equation for calculating the real time - in minutes or hours - it will take your pet to lvl so you can be AFK and secure in the knowledge that your pet is doing just fine (unless, of course, you get DCed):

(total exp needed to lvl)(3.5)/60 = number of minutes.
Divide by 60 a second time to get the time in hours.

Example:
Say you want to train your pony from lvl 2 to lvl 3. The formula for the time it would take to lvl would be as follows:
(712)(3.5)/60 = 41.5 minutes

Similarly, if you wanted to train your drake from lvl 28 to 29, the equation would be as follows:
(6700)(3.5)/60 = 390.8 minutes; 390.8min/60 = 6.51 hours

This formula, coupled with the XP chart above and the Useful Numbers displayed below, will help you calculate how often you need to check on your pet with the utmost precision :)



Useful Numbers

Total Experience Required (1 -> 37) = 154,410 points
Experience Gained Until Starved = 8,571 points

Note: The latter assumes the pet is no more than one level higher than you.

Pet experience done by parrendar, Pimorez & Graut.