Plan a time-blocked schedule.
Time blocking works only if the blocks and the gaps between them both fit in the day. Three 90-minute blocks is four and a half hours of deep work, and with 20-minute buffers the commitment is five hours and ten minutes — leaving under three hours for meetings, email and everything else. Seeing that residual before the day is booked is what prevents a plan that collapses by mid-morning.
Block schedule against the day
Deep minutes = blocks x block length; buffers = (blocks - 1) x buffer; scheduled = deep + buffers; remaining = workday minutes - scheduled
Sixty to ninety minutes for most people. Shorter than 45 minutes rarely reaches full concentration; much beyond two hours, attention degrades and the last portion is low quality.
Three to four hours is what sustained-attention research and practitioner accounts consistently suggest as a daily ceiling. Plans assuming six or seven hours fail not through indiscipline but through cognitive limits.
Because blocks overrun and transitions need time. Scheduling blocks back to back guarantees the second starts late, and by the third the schedule is fiction — which is how time blocking gets abandoned.