Long answer, as this is from the pure philosophical POV. For some of the contents, I have the verses and for some of them it's inferred. Refer below as a pre-requisite:
Are there any scriptural references of the trinity “Brahma, Vishnu, Mahesh(Shiva)” as depicted in popular culture?
3. "If Brahmdev life span is 100 years then is it true for Devi Saraswati?"
Lifespan is only a Day for Brahma. Or 1 iteration to be precise.
(Day = morning + night)
For self similar patterns, we may use term "cycle". But for not so similar repetitive patterns, I will use word "iteration". e.g. my previous birth and this birth are different. So I will call it 2 iterations of "iammilind" and not 2 cycles.
I interpret "BrahmA" (or Brahma-deva) from philosophical POV, as Saguna Brahman (whole universe). Universe as a whole can have only 1 iteration.
Now like you & me, if we consider the whole universe or saguna Brahman as a "being" / Purusha, then it dwells in nitya sattva (continuous illumination). This term is used in BG 2.45.
As the sattva increases the longevity increases. And hence, the time between the beginning & liberation decreases. For example,
- The time between beginning & end of Universe is only 1 cycle. The sattva is at highest. This 1 cycle is referred as 1 day of Brahman. In this case, 1 cycle will be same as 1 iteration of universe life:
1 human Kalpa (thousands of Yuga-1) = 1 day of Brahma
- The time between beginning & end of stars/suns would be more than 1 iteration; These are called supreme of deities (Surya deva etc.)
Centuries of human Yuga-s = 1 day of Sun
- The life time of earth like planets (Bhumi devi) is lesser than Sun etc. and hence it takes few more iterations compared to stars
Several human Yuga-s = 1 day of earth
- Life times of various natural elements like rivers, land, wind, seasons etc. is now divided in even more iterations; However these are still divine compared to humans due to its sattva nature;
1 human year = 1 divine day
- Life times of pitru are many more iterations;
1 human month = 1 ancestor month
- Life times of humans and equivalent animals is average. Hence they have iterations in form of several Yuga-s compared to Brahma
1 human day = 1 human day
- Life times of some reptiles & other animals are even less; Hence they would be finishing even more cycles; Tamas (ignorance) beginning to show up, from here predominantly
1 human decade = lifetime of animals
- Life times of other small insects are lesser than these smaller animals; They take millions of iterations till Moksha
1 human week = lifetime of these insects
- Life times of bacteria & virus are fractions of seconds and hence they would be finishing huge number of iterations.
1 human second = lifetime of insects
Some of the scriptural sources.
Note that at top and at bottom of this list, it's "Me" or true "Self", where there is "no time". i.e. already in the state of Moksha.
2. "What happens to his wife Devi Saraswati after death of Brahmdev?"
The Saraswati here is a symbol of "Knowledge" or illumination. Knowledge of Brahman (or Self) is revered throughout Bhagavad Gita, so not quoting them here.
Knowledge of Supreme = Saraswati-Brahma
At the end of Brahma's morning. Everything merges back to the nature of the "Self/Me". Hence there is neither ignorance nor Knowledge. No space, no time. It's pure nothingness. So Saraswati merges back to Brahma and meets the same end.
1. "How does he die?"
"Die" is probably a wrong word. "Brahma" is Not a person or body. Here we refer it as a whole universe's consciousness (Dehi). The bacteria and cells on our body, has their own consciousness. But our body has a whole, its own consciousness. Similarly, the whole universe, which I called above as Saguna Brahman has it's singular consciousness.
BG 2.30 - O descendant of Bharata, this embodied Self (देही) existing in everyone's body can never be killed. Therefore you ought not to grieve for all (these) beings.
At the end of the morning (or day), the consciousness merges back to the self. For the living beings, it may appear like a Pralaya.
BG 9.7 — O son of Kunti, all elementary entities enter My nature at end of the cycle(Kalpa); Again I create them at the beginning of the cycle.
4. "As there is new Brahmdev after 100 years likewise is there new Devi Saraswati."
Exactly same universe (BrahmA) and its knowledge source (Saraswati) would be born. No memory of previous cycle to retain the same similar pattern. Exactly same events and exactly same end. This is called Kaala chakra (Eternal Return). Please refer below post:
When Brahma's day ends and a new day starts does the previous day repeat itself?
Note: IMK, many people don't agree to this concept. Please use your conscience before subscribing to above philosophy.