How can i convert a short time high voltage signal into a long time low voltage signal?

You need to charge something like a battery or capacitor that you can drain slowly.

easy by using a couple dropping resistors in a voltage divider and a diode to the trigger of a 555 timer IC chip. Very common chip less than a dollar. The voltage is dropped and turned into DC by the dropping resistor network and diode, since you did not say whether it was ac or dc.
The timer chip is triggered and the output stays on as long as you want using the right value of parts which you can calculate using the online calculator that usually are found on sites that have these diagrams. Just use yahoo to search for 555 timers, and try typing in high voltage triggers etc. the chip can also turn on a relay if you like.

Since you did say Voltage signals I assumed you were talking about ac Voltage rather than dc Voltage.

You can not produce a continuous low Voltage signal output from a limited time high Voltage signal.
Unless the short time high Voltage signal  repeats it`s self to sustain the low Voltage signal then the low Voltage signal will die no matter what kind of conversion circuit you have. For instance a step down transformer`s low Voltage output signal is no longer present across the secondary winding once the high Voltage signal is removed from the primary winding.

I'm sorry but I don't have a clue in terms of short or long time voltage transformation. However, if you want to reduce a higher voltage to a lower voltage, you would need a transformer.