Here in the UK exposing cyanotypes during the 'daytime' works fine (note I carefully avoided mentioning the sun); it can just take a fair bit longer! With really dull days, it could even take a few hours. You may notice that the contrast varies with different exposure times; but primarily just expose by inspection. Luckily cyanotype is cheap enough to just try it out and see what happens. I have found that using an ordinary light meter against a the same wall every day will give a good indication of how much UV light is available; i.e. if the meter read LV13 on a sunny day and exposure took 1 minute, and it is LV9 today, there are 4 stops less light (in the visible band) . Therefore a good guess would be 4 stops longer exposure (x16). I would therefore have a look after about 11 minutes and it should be about a half stop under exposed. I would then check every minute, expecting the 16 minute mark to be quite close.