Magnetization measurements in La_0.225Pr_0.40Ca_0.375MnO₃ allowed us to construct an H-T phase diagram of this prototype phase-separated manganite. Magnetic relaxation results clearly demonstrate the dynamic nature of the phase-separated state. Our results showed the existence of large relaxation effects, and the relative fraction of the coexisting phases rapidly changes as a function of time. Through a thermodynamic phenomenological model, it is possible to interpret the experimental results, both static and dynamic, of the phase-separated system. The phase diagram resulting from this model is presented, including the static and dynamic properties characteristics of the phase-separated state in manganites.