There is no official maximum output pulse length for a monostable multivibrator device, but there are practical limits.
The leakage at the Rext/Cext pin will limit the resistor size. For example, with the SN74LVC1G123 the maximum leakage is listed as 0.25uA in the datasheet. In order to keep the voltage drop at the output minimal (10% of Vcc), the resistor cannot exceed ~660 kohms (R < 10%*Vcc_min/I_I_max = 10%*1.65/0.25e-6 = 660 kohm).
There isn't a maximum limit on capacitor value, but 10uF is a realistic maximum limit for surface mount capacitors.
Using these two values, we can estimate the output pulse width (K = 1) as:
tw = 1 * 10u * 660k = 6.6 s
The application report Designing with the SN74LVC1G123 Monostable Multivibrator has some useful information regarding designing with these devices.