Generally very little downtime - somewhere between a few minutes and an hour-or-so, unless there are specific problems with your install or your ISP stuffs up somehow.
I switched from an LLU ISP (Be) to FTTC with A&A. The whole thing took less than an hour and that includes the installer nailing the VDSL modem to the wall and fixing up the cable to my master socket. the connection via Be worked fine until the switch-over, the connection via A&A worked fine immediately after.
If you are planning to need your connection for essential stuff such as you use it for business purposes: be paranoid and assume it could be out for the whole of the install day, and have alternate connectivity ready for those essentials.
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Current Line: Andrews & Arnold (AAISP) via 80/20 FTTC, getting close to the full rate both ways, joined July 2011, upgraded from 40/10 to 80/20 May 2012.
Previous setup: Be Pro with UploadPlus (ADSL2+, AnnexM), 12ish Mbit down, 1.6 up, happy customer for ~2.5 years.
Edited by dspillett (Tue 19-Jun-12 14:31:54)