Tank Lights Photoperiod

Tricky question this one and hoping you guys might be able to offer some guidance...

Question is:  when trying to determine how long to have tank lights on per day, how much allowance should be made for natural daylight...?  To be clear; I do mean daylight and not direct sunlight... 

The room my tank is in is roughly west facing (more like west south west) and gets quite a lot of natural light in there...  Currently, I have my lights on for 7.5 hours per day, on a timer, but can't help wondering if this might actually be too long when considering that the room itself is fairly bright anyway...?

Thanks Matt... 

The timer on my tank is a one shot sorta deal, as in once on and once off in a 24hr period...  Kinda makes the siesta plan a bit tricky (and I don't trust myself, lol)... 

I am wondering though if 7.5 hours is just a fraction more than needed and it sounds like it could well be...  Perhaps a good plan would be to slowly work it down to seven hours...?

Split the difference in the end...  reduced by 15 mins and will do the same next week... 

Hard to say which algae it is precisely...  It is very fine, like hair algae, but not as prolific...  Looks more like the Staghorn, but not as chunky...  Similar colour to BBA, but entirely different shape etc... 

Am hoping that having gone back to using liquid carbon at the moment - which I can't tell you how reluctantly that decision was taken - that this, combined with the shortened/shortening photoperiod, will combat the algae even more...  The theory from day one of this battle was to get the plants out-competing the algae and there has also been some brutal trimming away of the worst affected leaves...  Once the inhabitants of the tank have acclimated to the use of liquid carbon, I'll slowly increase the dose to nearer recommended levels too...