Capacitor to hold relay energized?

That calculator will not work for your application and there is not yet one available on this site. However, you can find the values of the capacitor and/or resistor you need fairly quickly by experimenting. Start with a 5K µfd capacitor without the resistor as shown below. The resistor is only needed to bleed off the charge of the capacitor and will shorten the length of time the coil of the relay will stay energized. If the delay time is shorter than you want with a 5K µfd capacitor, try a 10K µfd capacitor until the duration meets or exceeds the desired duration. If the duration is exceeded, begin adding different value resistors starting with higher values at first to shorten the duration until you reach the desired duration. You could also use a variable resistor as an attenuator to dial in the exact desired duration once you've exceeded it with a large enough capacitor.

Substitute your pulsed output where the diagram shows 12V+ in from a turn signal. Terminal 30 is your output.

I'll work on a finding a formula so I can write a calculator for this purpose and post it here.