ControlLogix 1756-L61 PID Loop

I am in the process of implementing a PID loop for dosing to a water system. I would like to use an analog input for a water quality sensor as the process variable. for the control variable, my only option is a solenoid valve that releases the chemical being sensed into the water, a BOOL value. As the help file states that the control variable needs to be: "SINT, INT, DINT REAL", I'm assuming this is typically some sort of proportional valve, VFD etc. My question is, can I make the control variable a timer value that holds the valve open for shorter/longer periods of time and apply that to my BOOL solenoid valve?

Yes, using a timer as a %Time controller is a common method. Search this site for many threads about that topic. You should find some sample Allen Bradley RSLogix 500 ladder logic for that. Search for "timer for proportional output".

http://www.plctalk.net/qanda/showthread.php?t=83271

If you can program in FB you can use the SRTP instruction (Split Range Time Proportion). It is commonly used for heat / cool applications.
TIP: You need to watch the min on time to make sure the pulse is long enough for the valve to actuate.

Using pulse width modulation to control the valve is fairly straightforward but I would recommend looking at your application in more depth before you commit yourself to a control strategy. The sort of dosing you suggest usually gives a very non linear response so it is normal to use some form of feed forward driven by flowrate with PID used for "fine tuning" the result. This is a very common application so there should be lots of advice available on the web. If this is for potable water you need to be very sure about what you are doing before you start adding chemicals into it.