7 to 9. I highly recommend researching up to lvl 9, as it takes less than a year to research if you've got a rather large Empire devoted to research, and it'll shave a few months off the conversion time nonetheless.
Ooh, this brings back memories..... I once played as a semi-customized version of the Pexlerr, had a rather large empire with over half of my worlds
completely crammed with research facilities. In the end, things like Stellar Manip took like 3 turns to complete
Big problem though, I researched ALL tech levels by the '30s or so
so then I converted all my tech centers to Intel centers..... I still have the savegame, never played much further tho..... needless to say, if I start playing that game again I could win by intel alone
MILLIONS of Intel points WOOT!