To meet the 5-year rule for Roth conversions, again the measuring period is five tax years, which essentially means any Roth conversion is deemed to have occurred as of January 1st of that year (Treasury Regulation 1.408A-6, Q&A-5(b)).
Notably, since conversions must occur by December 31st in a given year, their 5-year period will always start in the calendar year in which the conversion occurs; for instance, any conversion between January 1st and December 31st of 2013 will count as a 2013 conversion, but anything in 2014 will count as a 2014 conversion (by contrast, a new contribution as late as April 15th of 2014 could still be counted towards the 2013 tax year).
Unlike the 5-year rule for contributions, in the case of conversions, each conversion amount has its own 5-year time period (Treasury Regulation 1.408A-6, Q&A-5(c)), and thus with multiple conversions there may be multiple different 5-year periods underway at once.
When withdrawals occur from conversion amounts, they are deemed to be withdrawal on a first-in, first-out basis under IRC Section 408A(d)(4)(B)(ii)(II), which effectively means the oldest conversions (most likely to have finished their 5-year requirement) are withdrawn first, and the most recent conversions are withdrawn last. (Overall, the ordering rules from Roth IRAs stipulate that withdrawals are after-tax contributions first, conversions second, and earnings third.)