Hy,
Maybe a noob question, but does exclusive cards from one set get inside boosters eventually ? If yes, is there a default period of time ?
Example: I’ve set my eyes on Gisela, the Broken Blade, but I’m a casual player, so there’s no way I’m getting her from events. Is there any chance I would get her from booster eventually, or this cards are exclusive only for events and for a limited time?
Thanks in advance.
As soon as a new set comes down, the exclusives of the former set will be available in booster packs. No worries, you just need a good amount of luck and some patience. Or money, as soon your desired card is available on sale. 
They will enter the booster pool when a new expansion comes out.
A lot of people have started the practice of hoarding their boosters close to a new expansion for this reason.
Exclusive cards are only exclusive for a period of time. Mythics are usually only available to those that purchase them outright (alongside runes, maybe some crystals, and sometimes a fat pack), while rares can be earned through specific events - pay attention to the rewards of each event to see what’s up for grabs early!
Once all of the cards that are marked exclusive have been made available early, either through events or special offers, after a few weeks pass, they should be added to the boosters like all the other cards. Which usually also marks the addition of a new expansion to keep the flow of cards going!
For example, Olivia Mobilized for War in the SOI set was a cash exclusive, but is now available though the booster sets for mana crystals.