A period of time as in the colloquial American English, Come sit a spell. I hear it mostly from the mouths of rural Americans in the movies. Does that reflect general usage? OED tells us this:
1. A set of persons taking a turn of work in order to relieve others; a relay, relief-gang, or shift. Now rare.
1593 Sir F. Drake Revived (1628) 27 Rowing in the eddy..by spels, without ceasing, each company their halfe houre glasse.
3. b. dial. and Austral. An interval or period of repose or relaxation; a rest.
Examples from 1863 to 1975 may be found in Dict. Newfoundland Eng. (1982).
c1845 J. Tucker Ralph Rashleigh (1929) xi. 146 Both men took a hoe and gave the children a spell.
1847 J. O. Halliwell Dict. Archaic & Provinc. Words II Spell,..pleasure; relaxation. Somerset.
1852 G. C. Mundy Our Antipodes I. xi. 363 Your carriage horses will be all the better for a ‘spell’, (a rest).
For the etymology OED says “Related to spell v.3, and perhaps directly representing Old English gespelia substitute (compare spala spale n.1).” Spale, an obsolete word meaning sparing, respite or rest has only one cite in the dic, from the wonderful old poem The Owl and the Nightingale.
a1250 Owl & Nightingale 258 Þu mihtest bet hote galegale, Vor þu hauest to monye tale. Let þine tunge habbe spale.
Gespelia is interesting, I wonder if there are any other cognates in English of this OE term.