What is the longest observed lifespan of a mosquito, especially of the species Anopheles arabiensis?
Mean lifespan calculations for Anopheles arabiensis range from 14 days (Karoki, 2013)* to 21 days (Yamada et al. 2014)**
*This Masters thesis tests efficacy of different drugs against An. arabiensis, 14 days is the mean control lifespan.
**These An. arabiensis are of a specific strain but no indication is given that the life span is different than normal An. arabiensis.
The CDC Anopheles mosquito page is another good source for information on Anopheles life spans:
The adult females can live up to a month (or more in captivity) but most probably do not live more than 1-2 weeks in nature.
How long does it take for Plasmodium parasites to develop and move into the salivary glands of these mosquitoes?
US National Institutes of Health:
Growth and division of each oocyst produces thousands of active haploid forms called sporozoites. After 8-15 days, the oocyst bursts, releasing sporozoites into the body cavity of the mosquito, from which they travel to and invade the mosquito salivary glands.
United States Air Force (USAF):
The length of the developmental stage in the mosquito not only depends on the Plasmodium species but also the mosquito host and the ambient temperature. This may range from eight days in Plasmodium vivax to as long as 30 days in Plasmodium malariae.
Wikipedia (Plasmodium falciparum) :
Over the period of a 1–3 weeks, the oocyst grows to a size of tens to hundreds of micrometres... The sporozoites then migrate to the salivary glands and complete their differentiation. Once mature, the sporozoites can proceed to infect a human host during a subsequent mosquito bite.
US Center for Disease Control:
After 10-18 days, the parasites are found (as "sporozoites") in the mosquito's salivary glands. When the Anopheles mosquito takes a blood meal on another human, the sporozoites are injected with the mosquito's saliva and start another human infection when they parasitize the liver cells.
Overall, it depends who you ask, but especially on the species of Plasmodium you're working with. I'd look for specific papers on the development cycle you're modeling and run with it.