Hey =)
So called lever bosses are particularly challenging because you cannot use your vocation summons. You mostly have a time limit of 10 minutes (depends on the boss and the amount of tiles) for 5 man bosses. There are lever rooms for ten people (King Zelos) and there is a time limit of 24 minutes, see Tibia wiki: King Zelos Strategy . Then there are also 15 man bosses such as the world devourer, the final boss of the heart of destruction quest or ascending ferumbras, the final boss of Ferumbras Ascending quest. The time limit is not accurately known to me right now since fights over 30 minutes have been observed already. The only way of checking the time limit would be to start a timer and just stop it once you get kicked out.
Edit: Here is also a good, related question regarding the times:
https://www.tibiaqa.com/11371/how-much-time-can-you-stay-inside-the-different-bosses-rooms
But at teleportation bosses such as Obujos, Tanjis and Jaul you can indeed use your vocation summons. You can also use the summon at werecreature bosses. For the werebosses you also have 10 minutes. But for deepling bosses you have exactly 20 minutes. The time starts once you killed the first deepling boss (note you can only kill the according deepling boss if you opened its chest before and did the coral mining mission). The spawn time of a deepling boss is around 5 minutes, depending on the firepower of your team you can kill one deepling boss up to 4 times. If you killed a deepling boss and leave the teleporter, you cannot enter it anymore.
And the maximum amount of summons you can have as a mage is 2.
Hope I could help :)
Feel free to leave a comment if something remained unclear.