The length of time that is too long to leave a child unattended depends on their age.
For children that are under 8 years old should not be left unattended even for a short time and kids under 5 should certainly not be left unattended even for a short time, other than when they are sleeping.
However if your child is sleeping, then it's okay to leave then unattended or in a playpen while you take a shower or go to the bathroom.
Children ages 9 to 12 can be left alone and unattended for brief periods of less than 2 hours.
And some 8 year old children can be left alone for brief periods of no longer than one hour to two hours and only during the daytime or evening.
Children ages 12 and above can be left alone for longer periods of time and often up to 12 hours.
Even children 12 and up that have a good maturity level can also be considered old enough to babysit other younger children or siblings.
And ages 16 and above can be left alone for overnight periods and longer.
Each child is different and each child has different maturity levels, and the child's ability to handle unexpected situations and good judgement also vary.
Be sure that the child can easily reach a parent by phone and are able to call for emergency services if needed.
You can begin leaving your child if old enough for short supervised periods to gauge the child's comfort level and readiness.
And if possible have a trusted neighbor come over and check on the child.
Although toddlers and children under 5 should never be left unattended, for even a minute unless during sleep or they are in the next room as you and you can easily get to them.