The problem is knowing how fast they learn. For a n00b this is basically impossible, and takes cost monitoring to notice their rate of learning. For a person with the basics of the language that they'll be using, and a tiny bit of experience in the libraries they'll be using, things get easier.
For everyone (n00b to senior) I would always recommend designing & breaking things down into no smaller 1/2 day blocks, where nothing went longer than 3 days (for inexperienced), but longer for more experienced. Senior devs should be able to be able to be trusted (to be accurate) for week-long blocks.
Use a Gannt chart (ms project or similar) and monitor the inexperienced, gauging their real-life times with their estimates, adjusting accordingly. Give semi regular feedback at the accuracy so that they (& you, the manager) are working with.
In one of my companies, the most inaccurate we were for any project was that we were late 2 days over 2 years. Build on trust, build on meetings and other time sinks. Include testing, sign offs, etc.
So, absolute n00bs need close mentoring so that the (senior) mentor can begin to gauge the growth rate... Which will be steep... But different for everyone.
It's very useful to break tasks down so instead of a month long "upgrade elasticsearch" story, there's dozens of day-or-less tasks to work on. Often it's not noticed how much effort goes in to creating these new smaller tasks. If you aren't extremely familiar with the targeted version of elasticsearch, you'll have to spend significant amounts of time investigating, planning, and writing.
You can get very high accuracy on estimates by spending lots of time creating smaller stories and encouraging people to over estimate. This may not be in the best interest of the project or the business.
That means the business is taking safe bets. Depending on the industry this makes a lot of sense. If you are in a small business this probably won't help you.
For everyone (n00b to senior) I would always recommend designing & breaking things down into no smaller 1/2 day blocks, where nothing went longer than 3 days (for inexperienced), but longer for more experienced. Senior devs should be able to be able to be trusted (to be accurate) for week-long blocks.
Use a Gannt chart (ms project or similar) and monitor the inexperienced, gauging their real-life times with their estimates, adjusting accordingly. Give semi regular feedback at the accuracy so that they (& you, the manager) are working with.
In one of my companies, the most inaccurate we were for any project was that we were late 2 days over 2 years. Build on trust, build on meetings and other time sinks. Include testing, sign offs, etc.
So, absolute n00bs need close mentoring so that the (senior) mentor can begin to gauge the growth rate... Which will be steep... But different for everyone.