Up to this moment I did not had any special needs for a fast power up, so the 1 sec pause worked just fine for me.

But I am pretty sure that each osc will have its own time to stabilise. So, for 18877, page 112 of the datasheet, says that there is a timer named OST (Oscillator Startup Timer) that counts 1024 cycles of from OSC1, if the Oscillator is set to LP, XT or HS (all these are external oscillators).

For any case of oscillator selection (internal or external), there is OSCSTAT register that displays the status of that module in the appropriate bits (page 122).

Page 603 shows the wake up time that any osc selection needs.

Other PIC have same data also, so you may look accordingly.

Ioannis