Given the following circuit, I would like to find the time constant \$\tau \$. I know that at \$t\rightarrow \infty\$, the capacitor acts as an open circuit, so no current flows in it. In an RC circuit, \$\tau=R\cdot C\$, so R in this case would be \$R_1 + R_2\$.
However, given the same circuit written with Norton equivalent, I get \$R=R_1 // R_2\$.
My question is: what is the right way to find \$R\$?