With an excess supply of nutrients, in how much time will the number of algae cells double in full sun?
I am presuming this a Blue Green Algae(BGA) Identify your algae

Growth rate for N1, N2 biomass at t1 and t2 time respectively, can be give as,

Growth rate:   $K'$ = $\ln \frac{(N_2 / N_1)}{(t_2 - t_1)}$


(source: csiro.au)  

So, by this equation it is easy to calculate the doubling time of your algae. 



For BGA normal doubling time is 6-8 hours.$^2$   

Source:


http://www.marine.csiro.au/microalgae/methods/Growth%20rate.htm
https://sites.google.com/site/biomonstaaar/algae