Depends how well the kid naps/sleeps - when my son was little I could get decent 2-3h naps to get things done (not always and as they drop naps you will have less of that time).
Once they start moving/talking it's not even that you have to watch them so they don't hurt themselves (you can get a playpen etc. for a safe zone), but they will literally drag you to play with them. ;)
For uninterrupted longer blocks of time you will, as others mentioned, need someone else to look after the child. Though again depending on your kids character, if they stay in the house and can see you, they might not let you work in peace even if someone else is looking after them. Depending on your finances and location you could try getting family to help, look for childminders/nannies, nurseries, etc. Each option has its own benefits and downsides.
We share bedtime duty, so on OH's days I get most of the evening to do work and tend to also stay up late. And OH often takes our son shopping & then to the park on Saturday mornings, so that's another longer block of time for me to work. Likely, without childcare arranged, you'll have to sacrifice something (hobby, social time, etc.) and even that might not get you to a "full time" level of hours. Babies are more time consuming that a full time job, and the care can be very mentally exhausting, so that's another thing to watch out for. ;) You never knew how tired you can be after a whole day of doing "nothing" ;) until you've taken care of a toddler.
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