With guitar, the old adage "No Pain, No Gain" does NOT hold true. You can cause yourself some serious damage if you continue to play after your wrist and forearm become sore/tight. What I do is play until I feel my wrist or whatever getting tense/sore and then I stop for a while.. like 30mins or so.. if the tension is still there after I pick it up again, I immediately put the guitar back down again. Repetitive Strain Injury (RSI) is real and can cause permanent damage and in some extreme cases, temporary, and in even more extreme cases, permanent paralysis of those muscles.

Now of course when you are starting out, it's going to hurt your fingertips of course.. but this is not muscle strain.. this is just your fingertips getting used to holding down the strings. Slowly, over time, your hands, wrists and arms will get stronger and you will be able to sustain longer playing sessions.. don't over do it.. it's just not worth it. 30 to 60mins a day is more than enough when you're starting out. My friend at church studies music at university and he HAS to keep playing even when it hurts a lot of the time. Recently he has been forced to take a semester off because of RSI.