As in "the timeslot given to completing the task". Would also be interested to hear if there's different usages for different scenarios.
According to dictionary.com, it should be two separate words "time slot".
This useful article on compound words offers the following advice:
Many of them are found in the dictionary and are not subject to our interpretation, our judgment, or our whim. Start with your dictionary before applying any other guidelines.
I would be inclined to follow that advice, and use the dictionary version: time slot.
It seems that time slot is currently most common, but that doesn’t prevent you from adhering to more (or less) traditional approaches.
Here is the google n-gram of the variants in use since 1940.
Both timeslot and time slot are acceptable. You will find an equal amount of dictionaries containing either.
Regarding the hyphenated version, there's been a general trend to move away from hyphenated compound words over the past 40 years. A common, historical pattern is that two words — say, time slot — will be joined by a hyphen for a time — time-slot — and then be joined into one word — timeslot. English is moving away from this and joining languages like German, where words are advantageously and immediately linked to one another.