stupid question but

I have a "my baby's firsts" book. One of the spots is for "I can sit up". When would you ladies consider you baby to finally being able to sit up? Like when they are no longer shaky siting up or when they can sit up for a certain time period. My baby started sitting up last Friday. He is no longer shaky but cannot sit up completely straight for more than a minute. The rest is of him slightly bent forward but not tripod sitting.