Periodic sync of data using Llama
I programmed Llama to do exactly that. 

You'll need two events:


3G auto on
3G auto off


and one Llama variable:


auto3G


Event 1 (3G auto on)


when airplane mode is disabled [prevents the system to work if airplane mode is enabled]
when mobile data is not connected [this ensures there will always be a break of 60 minutes after each (manual) data or wifi connection]
when your phone disconnects from any wifi network [same as above]
when variable 'auto3G' does not have a value of 1 [new variables don't have values; this also ensures that data connections are not interrupted when activated manually]
delay for 60 minutes [change this to your needs, e.g. 15 minutes]
enable mobile data
set Llama variable 'auto3G' to 1 [so the system knows data connection was activated automatically]


Event 2 (3G auto off)


when mobile data has connected [lets te system wait until data connection is established]
when 'auto3G' has a value of 1 [only if data connection was activated automatically]
delay for 45 seconds [change this to your needs and speed of synchronization]
disable mobile data
set Llama variable 'auto3G' to 0 [system reset]


this should cover your needs ;)