Well that rather depends on what you're planting - if it was, say, summer bedding, about 6 inches - if its a large shrub, e.g. Cotinus, about 3 feet, more if there's no fence between your gardens.

sounds like a political neighberly kind of question apart from leylandi which now need planning permission . bamboos basicly right and i would take her information thow it isnt written in granet excuse my spelling . ive seen and will see quit a few questions about bamboo growing threw from a nieghbers and what can be done . it is know coincidence that our local garden centre sells know more bamboo of any sort . i think bamboo is going to become a big big problem and the garden centre is distancing itself from what inevitably is going to be a huge problem sooner or later . ironic that its your avatar name bamboo sorry about that . personaly i love bamboo . it is the plant that is some things to all men and all things to some men . the first light bulb element was made out of bamboo .it has over 2000 seperate uses and is the most durable plant on the planet with hemp comming a not so close second . its the second quickest growing plant to only beaten bye bull kelp which can grow a metre a day . sorry im full of usless information with terrible spelling lol .