General guidelines in the US are that frozen meat will keep for somewhere between 6 and 12 months. I don't know if haggis would be different: genuine haggis is not permitted in the US.

I have had haggis, or what claimed to be, in Scotland, and I'd far rather eat it again in the UK than a pizza of any description. Every pizza I've ever tried there - at least 4 or 5 of them - has been utterly vile.