Check Michael Karas's user page (click on User Pages button on the left). He has a spreadsheet to calculate delay loop times. Very helpful. I also recommend you print off a copy of the 8051 instruction set that includes the # of cycles required per instruction. Beware that some "high speed" parts like the Dallas DS89C420 have different instruction timings.

If you need a periodic signal, use a part with a PCA (P89C51Rx2 for example) or use 8-bit autoreload mode for higher-frequency signals.

- Lee